I think $500ish including the parts sounds fair.
Listen to Musturd, grinding will take at least 1/2 hour to 45 mins, plus grinding the SFCs to make sure they are GTG. Ligning everything up, tacking in place, final welding making sure you don't burn down the car, then after it cools do some paint for rust prevention. Easy 4 hours with on/off the lift, doing it right anyway.
I've seen the $100 install jobs from some hack with a welder. Busted welds with no prepwork, welding them inplace while on jackstands, etc. IMO do it right. Do you have to spend big bux to get it done, no. But you gotta know people. Maybe you do the grinding and prep, buy the parts, have a buddy zap 'em in afterhours at his work for $100 and a case of beer, you paint 'em after he's done. That's how I'd do it. Quality work is expensive.