The cost of a Kilimanjaro climb can vary from under $1000 US to well over $5000, depending on your choice of route and tour operator.
If cost is your biggest consideration, you can wait til you arrive in Tanzania to arrange your climb, bargain hard with local guides for a 4 night/5 day trip up the Marangu route, carry your own gear, and refuse to give tips. This will also ensure that you suffer from altitude sickness, have a low probability of reaching the summit, and leave on bad terms with your guide. We think you would be much better off to spend a few more days on the mountain with a quality tour operator, and treat your guides and porters fairly.
The cost of your Kili trek will scale with the number of days you spend on the mountain. A significant fraction of the cost of a Kili climb goes to park fees, which are charged on a per-day basis for each climber, guide, and porter in a group. In addition, you will be paying more in salaries, equipment, and food costs for a longer trip.
More complicated logistics also make longer routes such as the Lemosho route and the Shira route more expensive to run. These routes require more porters to carry food and supplies for the extra days, possibly resupply porters to bring up more food partway through the trek, as well as ground transport to the more distant trailheads.
To some extent, you get what you pay for on a Kilimanjaro trek. A budget tour operator will probably serve only basic food, and their tents and other equipment might be in poor condition (or you might have to bring your own gear).
At the other end of the scale, we ate like kings at folding tables and chairs in bug-proof mess tents on our Kiliwarriors trip. They also carried a shower tent, and a portable toilet so that we didn't have to use the smelly long-drop outhouses (these were the envy of the other groups on the mountain, and more than a few clients from other outfitters tried to sneak into our toilet tent!) Kiliwarriors also carried a satellite phone for emergency use (and for calling friends from the summit),
as well as extensive safety gear such as an inflatable hyperbaric chamber for severe altitude problems.
The cost structure of the tour operator's business also affects the cost of your trip. You will generally pay more to book through a company that has to support a head office and marketing team in Europe or the US, and puts out glossy brochures and full page magazine ads. Some of these tour companies simply subcontract the trip to a local operator, so you're not getting much value added.
The official currency of Tanzania is the Tanzanian Shilling, abbreviated Tsh, but the working currency on Kilimanjaro (and in the tourism industry in general) is the US dollar. Local stores will use Tshillings, but prices in most larger hotels and tourist oriented stores will likely be in dollars.
Bring lots of US cash to Tanzania, and don't count on using your credit card. Many places (even large hotels) may not accept credit cards, or else charge a surcharge for credit card payments. We were quoted an exorbitant 30% commission for a credit card cash advance at one hotel.
In Arusha, many banks have automated tellers (ATMs), so you can withdraw Tshillings with your home bank card. Make sure before you leave home to set up your bank account for international withdrawals. In practice, it seems to be hit and miss which ATMs will accept your card -- you're better off bringing cash if possible.
Tipping is a well established tradition on Kili, and guides and porters expect to be tipped at the end of a trip. Many trekkers are uncomfortable with the situation, but remember that guides and especially porters are paid very little (if anything), and rely on tips to feed their families. We are amazed at how people might tip $20 to the waitress who carries food to their table in a New York restaurant, but balk at tipping the same $20 to a porter who carries their pack and gear for an entire week on Kilimanjaro.
You will be amazed at how hard porters work on Kili. Plan on tipping around 10% of the cost of your trip, and bring enough cash (in small bills) to distribute at the end of the trek. A typical group tip might be $20 per porter, and $50-100 for a guide (more on longer trips, or if they go out of their way to make your trip enjoyable).
Gifts of clothing or gear at the end of a trip are also greatly appreciated. The Gore-Tex and fleece clothing that we take for granted in America and Europe is hard to find in Tanzania, and far too expensive for the average porter. We suggest that you bring clothing that you don't need to keep, and donate it to your porters on the last day of your trek. Even something as small as a used pair of wool socks will be greatly appreciated.