I recently purchased a house on Long Island, NY and when i removed some cabinents i noticed an uneven floor. The house is on a cement slab and in one corner (use to be a garage many years ago) the floor gradually slants to that corner to a depth of 2.5" off level. I want to put laminate flooring down but do not want to deal with an unlevel floor. Is there a way to build up that much floor? Self leveling cement would be very expesive as a total option because it is about a 50sq foot area that needs to be level from 1/8th of an inch down to 2.5" and i know that type of cement does not cover that much area.

It is very common to find floors in garages out of level such as this. Fixing this can be easy.
To understand why this happend you must relize that this floor was never supposed to be used as a living space. The contractor simply laid one or two inches of gravel on the ground and layed another three or four inches of cement. He did not go through the process of properly compacting the surrounding foundation level, nor did he worry much about debris that may have been left on the ground. He simply covered it up. However you must understand that simply laying cement on the top will not fix the fact that it has settled and does not contain any methods of damp proofing under the cement. Also any additional weight because of the extra cement on top will only cause the older floor to move down more.
Thus with the leveling of the floor then placing any kind of new floor over of quality other then carpet, you will find the new floor will buckle and warp or stain if using roll flooring.

To properly fix this you will need to completely remove the garage floor. Not to hard but dirty. then Re-pour a new level floor with proper drainage (gravel) damproofing (plastic) and wire mesh to prevent it from cracking later on.
There is another method of fixing this called Mud Jacking. If the slab is in fairly good shape. Thick enough, and you have a large enough door to put in a wheelbarrow size device this can be done. What they do is drill one inch holes into the floor in various locations then use a pump device and inject a slurry of cement and clean fill dirt into and under the floor. The pressure caused by doing this will cause the old cement floor to rise to the height you wish to acheive.

I do however suggest that you completly remove the cement floor and do this properly otherwise you will be disappointed in the future when the floor begins to buckle and warp because of dampness coming up from underneath.

If your outside stud walls are sitting on this same slab then by all means DO NOT consider slab jacking. Do the walls rest on top of the slab? Are the outside walls level relative to the floor? If the floor has been crooked for a long time, it may not settle any further. Is there a crack in the floor where it starts the downward trend? What is at the perimeter of the slab?

Thanks to all who helped..i had some luck from the previous owner of the house, who lives next door now. It appears that it was just a bad poor job. I had a mason come in and for $650 he has done what he needs to do to level it out. looks great!