You can't reliably waterproof a basement from the inside. You can't reliably waterproof a basement 'relatively cheaply'.
The only sure way to waterproof a foundation and at the lowest cost is to properly waterproof it on the exterior before you back fill.
Barring that, you need to accept that fact that most basements may get wet at some point.
Given that, you need to do a few things:
- make sure you're not fighting a losing battle with a high water table. (if you are, then you need to look at water moving solutions such as perimeter drains and sump pumps (and back up sump pumps)).
- make sure you remedy all surface water issues. Make sure there is proper slope around the house, all gutters are adequate, working, and have a proper downspout, etc.
- build proper walls on the interior
Regarding the latter, a proper wall would be, from the outside in:
- foam insulation. Spray foam is ideal. Polystyrene (expanded or extruded) are more typical
- studs (I'd strongly recommend steel studs. See here: https://diy.stackexchange.com/a/8644/1209 )
- sheetrock (I'd strongly recommend going with a paper-less sheetrock product like densarmor.
What you won't see in a proper basement exterior walls are:
- fiberglass insulation
- plastic (no vapor barriers!)
- mold-loving surfaces (wood and paper)
The idea behind the 'proper' wall is that it assumes moisture may be an issue at some point--either from the wall itself, or moist air trying to condense on the wall. A spray foamed wall prevents all air movement and seals tight against the wall...this prevents moisture migration. XPS and EPS can't do that as well, but both can also breath to an extent, so moisture can eventually migrate as needed.
Avoiding wood and paper faced sheetrock also reduces the surfaces mold can attach to.
Note that even after all of that work, you will still want a dehumidifier in most climates down there as well running at all times.