Maybe you could have it cut on frosted vinyl (like stickers) and stick it to the plexi? Not sure what effect your going for, just throwin out suggestions.
Go to your local sticker shop and get them to cut you a reverse sticker, put it on the plexi and what ever you want to be clear tape up with masking tape...
Go to a machine shop and see if you can use their sand blaster on LOW air pressure...blast it, take the stickers off...and you have a 'frosted' panel...
I have access to a CNC laser engraver and could etch it onto a piece of plexiglass or acrylic. How big a design are you looking for and how big is the sheet or piece of plexiglass? My engraver is limited to 18"x24" pieces.
I don't have the materials so you'd have to send it to me for engraving.
If you have a large logo that would be helpful also, best if it's vector graphics logo.
The last thing I did was a piece of maple and the engraved area was 6x15 I think and it was about $35-40.