Leveling sloped basement floor
Option 1: As cutrightjm mentioned you could use a cheaper cement below the self-leveling cement. This might be the slimmest option, but I personally won't trust multiple thin layers of concrete not to crack and breakup due to their differences in thermal expansion. If you choose this route, do your homework on the expansion properties vs your existing foundation.

Option 2:  If you are willing to add 3-4" of cement to the highest point of your floor, you could just repour right over the top of your existing floor and then refinish it so it's level. This would likely be the easiest job to contract out.

Option 3: The easiest method would be to use a moisture locking floating subfloor such as DRIcore.


Such a subfloor can be shimmed to be level, and DRIcore even sells a leveling kit.

Option 4A: The most inexpensive route would be to attach GC (ground contact) treated strapping to the floor to act as joists and install a plywood subfloor over the top of it. You can shim the joists with composite shims, or cut the joists to slope.  For this you'd likely want your strapping to be at least 3/4 thick at the high point of your floor so you have something to screw into. 

Option 4B:  Similarly, if you are worried about moisture problems and want to reduce the floor height, you could use cement board instead of plywood. Cement board could be attached directly or shimmed to your floor at the high points. After it's screwed down you could level out any small imperfections between panels with some  that you could level out any small imperfections with thin-set.