Here's some facts
 http://www.gulfstreamacademy.com/firstofficer/firstofficer.html 
Log More Than 280 Hours Of Airline Quality Flight Training For Less Than $72 Per Hour
doing the math there that works out to you paying $20160 
although *After receiving $2,000 in flight pay, program cost is just $27,684
The overall program is expected to take approximately seven months to complete.
 
http://www.aviationinterviews.com/pay_rates/mesa.html 
Mesa B-1900  First Officer  $18.15 x 70 hours gtd + per diem of 1.15hr  works out to per bid earnings of $1270.5 or a 7 month total of $8893.50
in both examples you are doing the exact same thing, acting SIC on a B-1900.
using those two facts it's clear to see who benefits from which scenario, that is a difference of $29053.50.
that's 29k less the gulfstream FO has than the Mesa FO
it's also 29k that gulfstream saved over Mesa.
I used the 20160 number as that is what Gulfstream advertises as the cost of the hours and assuming the remainder of the 27k is for the 121 training since I wasn't able to find out if Mesa makes you pay for your training with them.
the question as to weather or not Mesa pays enough is for an entirely different thread, just illustrating why PFT is bad using some facts for Matt