can I get the whole attic insulated for cheap?
You can insulate the attic yourself for cheap,unless you live in a massive sq ft home you can do an r-13 DIY insulation job yourself for around $500,that still wont help your problem,you'll then need to ventilate the area.
Keep in mind that homes with an upstairs like bedrooms & such have design features such as added roof vents,ridge vents & soffit vent systems in place to keep air flowing between the heated/cooled interior & the conditions outside,there is more to it than slapping up some drywall & turning an attic into a bedroom.
If you just insulate the attic,then heat it you'll still have massive problems with condensation only this time they will be even worse,the condensate will now be trapped between the insulation & the roof deck boards.
What you can do cheaply is to build an insulated room within the attic out of 2x4's,insulation & plastic,then vent any heat that need be vented from the grow room directly out a roof vent or chimney,make sure the room has its own roof,if you use the home's roof as the grow room roof condensation will still be a problem.