You need to be more specific/clear with your requirement.
"need to transmit a signal" -- any transmitter that can tune to 2.4 - 2.5Ghz can do that. Will it serve any purpose or be useful? now that's the question.
"range of 1 mile" -- one mile through space, through fog, clouds, rain, sleet, snow. Through trees, shrubs, buildings, people, cars, trucks, concrete...???
And based on your handle--are you in the UK?
Do you want to do this for less than a specific amount of currency?
Nevertheless, here is a likely solution:
You want to transmit data, likely at a decent throughput, over a clear radio line of sight distance of one mile. (Trees, and virtually everything else mentioned to various degrees, will distort your signal and either degrade throughput or make the radio link altogether unusable).
There are a huge number of solutions that are not very expensive in the 2.4GHz, unlicensed, ISM frequency band. Most utilize the 802.11b protocol (or are backward compatible with it).
Some popular manufacturers are:
Some good sources for this type of equipment are:
Most all of those sites have tips, primers and other documents to help you define your needs.
If clutter (trees, buildings, etc.) ARE an issue, your best bet at this time is the 900Mhz gear from
or in a few weeks, Alvarion is planning to introduce their 900Mhz OFDM product.
Hope this helps you a little.