Well, here's what I paid for each flight course, for another perspective for you. I was considered to be a good student by my instructors (I.e. required minimal repeats and had minimal failures). What's going to make it $15,000 as opposed to $10,000, as you said, failures of lessons (aka "units") and incompleting units and repeating units (usually due to a failure of a unit).

This is what I did in my flight training:

FA133.1 Standardization Course $2993.90 - 4 repeats/1 failure/2 incompletes
FA133USF Extreme Attitude Recovery (required for me, but well worth it!) $1694.76 - 0/0/0
FA133 Multi-engine add-on Rating $5031.00 - 1/0/0
FA232 Instrument (single & multi) $19884.04 - 3/0/3
FA272 Multi-engine Commercial $5541.26 - 3/1/5
FA273 Single-engine Commercial add-on $2361.17 - 2/1/1 (Failed checkride)
FA420 Airline Flight Crew Techniques and Procedures (A320 sim) $2236.80 - 0/0/0
FA417 CFI/CFII $11554.68 - 9/0/21 (21 incompletes because the orals were usually not scheduled long enough to complete all items that needed to be covered).

That's a total of: $51,297.61

Depending on the degree you get some flight courses are optional. FA420 is optional and you don't have to get your CFI's here either to graduate.