Sounds like a good price to me, because I am assuming that the tractor has to be split. My guess would be you're probably a couple hundred dollars in parts and the rest is labor.
As long as it is a reputable shop, I'd say it's money well spent (so long as that is what needs to be fixed with the tractor).
UPS just delivered a rebuilt clutch (pressure plate, disc, throwout bearing) today for my 1951 Minneapolis Moline BF tractor that I'm restoring. Cost: $270. There's a $30 core refund on the pressure plate and disc if you send the old parts back to the supplier. This is a simple Rockford RM single disc clutch for a tractor with a 27 hp gas engine.
I assume that your 3840 has to be split to do the clutch job and my guess is that the clutch pack is more expensive than the one I just got.
So considering the labor involved, $1200 doesn't sound bad.
Price is not bad. I assume you have a dual stage clutch. No sure if you want to tackle it or not but this can be a do it yourself project if you have a buddy to help and a free weekend.