Hi, I have just bought an 81 Fiat Spider 2000. The edges of the rear fenders are rusted and someone had previously covered them over with red bondo. I have two 81 rear fender patches. They are 6" ? inches wide and it looks like you can cut out the rusted area and weld these patches to the fender... I got a price of 600 a side to do this. Is this something I can do?

I'm curious what the scope of work is that they want $600 a side for? Like... finished with the quarters repainted?
If you are anywhere near south Florida I'll weld them in for free and you can do the finish work

Yeh that was at a local Maaco. What do you think a local body shop would charge if I clean up the rust. I would clean up the area so we could get to good metal. The shop would weld in the patch, do body fill and primer. What's reasonable ?

Working backwards, $1200 @ $75/hr is 12 hours and $300 in materials
Grind and cut away rust- 2 hours
Weld in patches- 1 hour
Fill and sand- 2 hours
Prime sand prime- 2 hours
Mask and paint- 2 hours
Clean/wet sand/ buff (not happening at Maaco)
Wash-.5 hours

Either my hourly rate is low, or your local Maaco knows that there are three layers of metal in the rear quarter which may need work or, they work really slow.
I'm not trying to throw them under the bus but it sounded like the price you got was to weld in the patches and it seems more like a price for finished rear quarters.

299. Will buy you an Eastwood 135 mig welder (on sale) plus a cheap auto darkening helmet and cart (Harbor Freight, 80. For both) plus a gas bottle (100. Or so). So for less than 500. plus some sheet metal you can fix that and any other sheet metal issue that comes up. Plus when you're done you'll have a neat new tool you'll find indispensable.
This is exactly what I did and I never touched a welder in my life.
Good luck.

I'm with Fred!
I did a little welding in college about 100 years ago but forgot everything until I started working on my project last year. Welding is not rocket science!