There was a bug in the manual lease extension calculation that was fixed some days ago, it's correct now. (automatic extension was always calculating correctly)
Tested on your 737-200adv, age 7yr. Lease remaining 4 months, current lease 190k. Extend by 1 year, new lease is 199k. Extend by 8 years, new lease is 170k.
If you originally got a very good deal for the plane when ordering new, then the extended price may be considerably higher than you paid previously. Depends on the deal you got the plane, inflation, and the popularity of the type (among others).