How is a torque converter a couple of grand? A newer beefed up tranny would be a couple of grand, but a torque converter custom made would be between $300-$600 dollars depending on what kind of options you want like anit-balloning plates, bronzed furnaced fins, blah blah blah. Just because you get a different torque converter doesnt mean you need a new "beefed" up tranny. You have to pick and choose your battles. The stock torque converter for a N/A Bonneville I think stall'* at 2000 rpm. The */C versions are at 1900 if I remember correctly. You could move it up to around 2500 closer to the torque peak and still be ok for city driving. But it also depends on the options you do on your car. If you don't have many mods on your car, do other things first, not a torque converter. What kind of Bonneville do you have and I can make other suggestions, let me know what your goals are.
ADTR 1999 Black Bonneville SE
2003.5 Yellow Mazdaspeed Protege
2003 Azure Blue Mach 1 Mustang