If you have your own land then ponies are pretty cheap to keep, and as Simon says it is more a case of keeping the weight off than putting it on. As they get bigger, so does the cost of keeping them, it is the small native ponies that are the really hardy good doers. The estimate given out by the British Horse Society is that you need to allow an acre and a half per horse for grazing, this is less for ponies but you need enough land for them to get a reasonable amount of pottering about space and to rotate every so often. The land will also need to have the poo picked up at regular intervals unless the land is enough acreage for lost grazing area to not matter or you can harrow it and rest it for a few months.
As has been mentioned above, they are great fun and definately to be recommended but not to be entered into without knowing a little about their needs - especially if intending to train them. The usual recommended minimum length of time for a well handled pony to be broken to drive is 4 weeks, that is for basic straight along the road driving and then obviously you will need a bit longer for other jobs and to get a safe and reliable animal.
I have been driving Shetland ponies for 17 years and can heartily recommend them as great fun and good for doing jobs around the place..mine get roped into all sorts of things along the way, as well as doing the show stuff (and we aren't posh we just have an exercise cart, nothing flash). I also drive them as a pair, tandem (one in front of the other) and unicorn (two behind, one in front), they are a very cheap way to have a go at a fun sport! With my pair when they were very fit I used to be able to do 25 miles in an afternoon (about 4hrs with a 1/2hr break in the middle) with two adults up.
Here is my 22yr old proving he still has what it takes to go fast at the show this summer!
) so I have to drive of cycle to him.
