I would think it would depend upon whether or not the prior bottom job was ablative paint
or not. Mine is ablative so when its due for another bottom job all I really need to do is pressure wash and give it a good scrubbing with a stiff bristle brush, after all there's really nothing left to sand except gelcoat
and/or barrier coat.
If you do need to sand, you have a couple of choices. One you gather your friends (the ones that you were going to dump anyway) and have a sanding party. Bring some beer
and toss in a pizza or two and you're golden. Or golden until the next time you do a bottom job.
Another option, is hire someone to just do the sanding. At the yard I go to there is always someone eager and willing to pick up a few extra bucks and some do fine work. You'll have to negotiate the price
but around here I suspect $50-100 will do the job.
Anyway that's my 2-cents. I wish you hadn't posted this question though because I am now overdue for a haulout and bottom job and I was trying not to think about it.