SBT used to have an unlimited user plan, .i.e., you paid for the source code and then you could use it however you wanted, short of reselling it. Those days are long gone.

Quickbooks is a fantastic little program, and although there's no source code option there is a COM (or XML) interface in the current version that I've used a few times.

There also used to be a program called My Accountant, similar to the SBT of the time (maybe 1995?) when I looked at it. I think it was $795 with source back then.

The bottom line, you're right, they're exorbitant, particularly in my experience from which I have found that beyond basic accounting it's not a particularly notable program.