its a big job, $300 sounds about right to me. I did mine myself ( with the help of my dad - mechanic ) and we ran into a few different obstacles. You can pick up a good 3sp for around $100-$150 ( think Jack Travis ) so $300 on top of that isnt a bad deal considering the price of a good 4sp.
Problems we ran into were the mounts wouldnt line up so we used some "tongues" to allow the box to be bolted to the subframe. Also the wiring is a little different, I think we had to rewire the neutral safety switch ( inhibitor switch )
tailshafts are the same etc
EDIT* I just realised that you already have a 3sp, just replace it yourself as that price is ridiculous, its a fairly easy job.
Lets go find that Hotdog tree I planted
Lowered EF Futura
Black Steelies ( too cheap to buy proper mags )