Personally I do not like the electric sanders. It is quite possible that is what has caused the problem in the fist place. They are ok if used by someone who is very experienced in knowing just how much sanding is required. See the post below for details of what happens when plasterboard joints are over sanded.
plasterboard ceiling
To fix your problem you will need to re-trowel the joints with top coat. If the joins are over filled you will need to trowell them out twice the width of the trowel Preferabley use a 12" blade.
Sanding does not need to be a difficult job. Use a wet sponge to sand the edges then a hand sander through the joins. Using a hand sander allows you to feel where more sanding is required or less sanding. The sanding machines just rip off to much and you have very little feel for what is happening, in my opinion.
Just the other day I was at one of our projects where an inexperienced guy had over sanded the job. The contractor had to go through and re top coat the entire job.
As far as you question is concerned there is no cheap option in plasterboard sanders. However most plasterboard sales branches hire them out if you deciede to go down that route.
Cheers Rod