Well, I got my skimmer from where I work, the PetSupermarket (ughhhh*shiver*) It is a very crappy one, a visi-jet, it is rated for 70 gallons, so that would be good for you-- I think that they are 42.99. I personally wish that I had gotten a nicer more expensive one, but if you're on a budget that might be the way to go...
The most simple protein skimmer is a plastic funnel connecting to an air pump. It removes the organic matter by the surface tension of bubble. It just cost about 15-20 US $. Suitable for most small to mediate size tank.
I have a prizm pro deluxe that I just love.It works very well and has a basket in it that will hold alot of carbon.I think brand new they are a couple hundred but you can usually find people selling used ones for very good prices.I got mine for $50 because it had a crack all the way down one side and I glued it back with some acrylic glue and have been using it every since.
In my opinion a protien skimmer is the last thing that should be purchased frugally especialy for an octo. If it means having to wait a little longer to get the guy, I would opt for the very best skimmer you can respectively afford. The Aqua C remora or urchin would be an affordable good choice.