Prorata
Bond :15,000 USD.
Contract 5 years.
If you leave at the end of the first year, you have to pay the 4 remaining years 15,000 / 5 years = 3,000 * 4 years = 12,000 USD to pay.
If you leave at the end of the fourth year, you have to pay the 1 remaining year 15,000 / 5 years = 3,000.
Since there is no mention, I wonder if you will have to pay the entire sum (15,000 USD) as it is done in some countries (where the common rule is to take advantage of people).
It is definitely a question to raise.
Now, I paid 9000 USD (including accommodation and transport) for my first type rating in the US in 2004. It seems to me that 15,000 is a bit expensive, but that includes the line training...
It is much more expensive when it concern a conversion course (meaning the joining pilot is already rated...), therefore just a "transformation" to the company standards.
It is definitely a way to lock people in the company.