Frustration with numbers...
English numbers always turn out in my mind in their number forms and I understand them. The only problem for pronouncing numbers in English I've dealt with is distinguishing 13 & 30, 14 & 40, 15 & 50, etc... seems that we'd pronounce 40 as 4D instead of 4T when it's spelled forty. However, we'd just write it in its numeral form. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Now here’s a point to make:
Why not mention to Spanish learners how incredibly "easy" it is to "learn" how to count to 3?............. Uno, dos tres.............. Everyone knows that! My experience with "learning" how to simply count to 10 wasn't anything better than repetition and struggle. Great! Now I have little trouble hearing native speakers say single digit numbers except that in my mind a number doesn't appear!!! With a little effort, I understand what the numbers are... unless they're speaking of numbers in the 10's, 100's, 1000's, or even higher!!! "Learning" the cardinal numbers had been difficult enough to learn for the first time when I was a beginner in Spanish class and they won't stick in my head even with repetition and practice. I'm only using numbers in English as a native English speaker daily so it's no use! I can't comprehend large Spanish numbers without thinking until my brain aches. I get the same problem when reading numbers. I've "learned" the irregularities in spelling Spanish cardinal numbers but yet I still can't recognize them upon hearing them! All that shows up in my mind is just spelled out words! Agh. This is still a problem and I'm at an intermediate level of Spanish knowledge. Does anyone else have this problem? 
5 Answers
I think the problem people struggle with when learning a second language, is that we expect too much too soon. Think back, if you can, to when you had to use your fingers to display how old you were. When learning to speak our native tongues, we were not burdened with dealing with math at the cash register, etc. We learned gradually. Also, when we teach children numbers, we usually give them a visual picture (a picture of five apples) along with a visual of the #5. All sorts of tools are used to help children to "picture" and "hear" the numerical symbol.
With Spanish, I started out with a smaller group of numbers, namely zero through sixty. I did this by learning how to tell time. I have mentioned this before, but there was one thing that really helped the most. Every time I looked at a clock or watch (which is quite often) I would tell myself the time in Spanish. This helped me to not only fixate on the number symbol, but also on how it sounded. I would also tell myself the date in Spanish several times a day. Eventually, this practice grew to trying to read numbers where ever I went. This time last year, I was planning a missions trip to the Dominican Republic and knew I would be called upon to give Bible references (chapter and verse numbers) along with helping in a music program (more numbers). This pushed me to really focus.
Once you have enough practice with zero to ninety-nine, adding the hundreds and thousands becomes much easier. This becomes really important when shopping. And yes, those wonderful calculators usually come out, but after several months of focused attention, you will not be needing them if you are ever shopping in Latin America. 
When you are reading a word in a second language, you see the word for what it is - you see it and think about that word in your head. I think when learning numbers, there is a tendency to read and think of a number (the ordinal, not the spelled out word), you tend to think of the number in your own first language. I've been trying to be very conscious of thinking of the numbers in Spanish, but I have to keep reminding myself.
I think everyone has trouble learning numbers in a foreign language. Maybe they are just not "visual" enough. In other words, I can learn to associate a concept with a noun or verb so that I don't need to translate constantly into English. But with a number above 3 or 4, I just can't do that (studies show that we can't visualize numbers greater than that without counting).
Lots of practice helps, of course. Like saying every number you run into in Spanish, or even going through the phone book and saying the numbers.
I used to have trouble with seis and siete. Now I associate the final 's' in seis with the curvy shape of the 6, and the 't' in siete with the shape of the seven, and its surprising how much easier that is for me.
The only time I got really good at numbers was when I lived in Germany for a year in high school. I can actually think German numbers. But I can't think Spanish numbers, not yet, anyway.
Good luck!
It took me a long time, too. Often when I'm shopping, a clerk will tell me the price of something, and I'll get something of a puzzled look on my face (probably I missed part of the number due to not hearing it well, rather than that it doesn't convert in my head). Many clerks will pull out their ubiquitous calculator and punch in the numbers for me.
I still have a difficult time distinguishing between tres and seis and between setenta and sesenta. Again, not because I don't know the difference at a viseral level. It's because I can't hear the difference. This is a function of being around too much loud noise when I was in my early 20s.
We don't use numbers in our speech nearly as much as we use other words, and numbers are less subjective. Just be patient. With exposure and practice, you'll start to think in Spanish numbers.
And I thought I was the only one having problems when buying something! I feel better to learn I'm not.
I know the numbers, but cannot "see" them when a price is presented.
Frustrating!
It just takes time...
