Well the price is definitely right for the powder coating, my local powdercoater is sketchy to give a quote and mentions figures around $500 to do a frame including the blasting. Needless to say, I stick with paint. I have a YZ100 that came up very well with a scrub like your RM. It has been used by my son and I since the scrub up and now that the bottom end needs work, it is getting a birthday with lots of NOS bits and fresh paint for the parts originally in gloss black. It had surface rust and some pitting on the frame and arm but all tabs and brackets secure, no rail damage and footpegs straight with little play.
The YZ will be ridden as it is too much fun not to.
I will need to replace the rims as they are cracked in several places front and rear, I suspect the corrosion under the bead will be severe as well. The forks have mild corrosion at the top like most aging chrome and will stay as they are this time around with just a service.
The engine is getting a NOS crank assembly, new clutch plates, new piston and ring plus all the usual gaskets seals and bearings. I have glass bead blasted the top end to remove all the flakey crud and the engine will get a new coat of satin black.
I found it languishing away around the side of a friends house and rescued it. I couldnt believe the way it loĆ³ked and that just a few days of replacing nuts and bolts and a good lube up, she fired straight up and ran so crisp and clean. A real diamond in the rough as they say. Super satisfying to revive as it is a little rocket.