An open aluminium 20' boat is a lot lighter than a 'glass cabin cruiser. When you say 'only 20', you need to clarify that.
As for the winch, have you tried putting a sheave and hook and making it a double line pull? That reduces the effective load by a bit less than 50% (still have the frictional losses of the sheave)...
What winch are you currently using?
As for a winch less than $300, yes there is probably some available, but they aren't going to be particularly good quality, or last very long at that price.
You get what you pay for, and (my favourite), cheap no good, good no cheap. My wife's favourite, and from her native country, 'The foolish man pays twice'.
Chris.........