I have recently done a lot of research in to guides for doing Mt. Kilimanjaro here are some points I found that were important.
There are three types of tour groups low end, middle and high end. TheIn 2011 the price for low end iswas roughly 1800 - 2100USD middle is 2100 - 3000USD and high end is anywhere from 3000 - 5000USD.
You'll find that the high end groups usually do not do much more than the mid range groups to justify the cost.
I would also not reccommend using a low end group as corners are cut especially when regarding safety.
One of the main things to consider is how long the trek up is. If it is 6 or less days then your chances of making it to the top are greatly reduced, it is somewhere in the vicinity of 60%. For each extra day you add for acclimatization to the altitude you improve this 60% chance by roughly 10 - 15%. However this is subject to how well you adjust to the altitude which is completely random.
Another point to consider is how well the porters are treated by their guides/the tour operator. Look out for an operators that is a member of the 'Kilimanjaro Porter Assistance Project' In this way you have some assurance the porters are treated well and paid accordingly for their work.