I see a lot of people using the walkie talkies that are on the Family Radio Service Frequencies. The pros of this system are that the radios are fairly cheap, I've seen pairs for less than $30. But, the downside is that the radios don't have very good range (They say up to 5 miles, but I've never seen a pair of them that could go even 1 mile), and in WDW, it will be hard to find a clear channel, because so many different people are using them.
It will cost significantly more to move up to business grade radios, but the business grade radios will have the range you need, and there will be far fewer people using them.
I'd look into the cell phones that have walkie talkie features built in. Nextel has the best system for this. (I believe Disney is actually switching to Nextels for their in-park radios). If you don't already have Nextel phones, and you don't want to have to sign a contract, they do have a pay-as-you-go wireless service called Boost Mobile. They offer walkie-talkie only service for as little as $1.50 per day. This may be your most affordable option for good coverage. (the issue, of course, is the cost of the phones ... looks like the cheapest model they offer is $60 each)