You should fill your location in, in your profile so we know where you are
But box filters and UGF (under gravel filters) run off air pumps so are very cheap, a box filter is just a pastic box filled with filter wood or sponge air goes in the bottom and draws water up with it, UGF are plates that go under your gravel air goes up tubes sucking water through the gravel.
Box filter are easy to fit if your tank is already setup, but don't work as well as UGF. The next step up is an internal filter which it sound like you have already, these work better again.
The price doesn't really change that much, air pumps $10-$15, plus box filter $10, or UGF plates etc $15-20, internal filter $30+ But its been a while since I purchased any of these sort of filter so I'm sure someone will chip in if I'm too far out