The overriding factor should not be the cost of use. Instead, take into account a combination of the best overall installation for the type of existing construction, its installed cost and then the cost of use. While a wet system is probably cheaper to run in that you are already running the central heating in other parts of the house, it is generally more expensive to install in small areas and tends to work best when embedded in a screed because this best dissipates the heat. This is especially important if you propose any wooden floor finishes. If your floor is a traditional suspended timber joist construction with floorboards you can lay a wet system over this using a pre-cut insulation system where the pipes are incorporated within the depth of the insulation with a floating chipboard floor laid over, but this would probably raise the floor by around 75mm. There are a lot of electrical underfloor heating options available for both screed and dry finishes, but again these need to be chosen carefully based on your existing construction. Be careful that you do not go for an electrical undertile heating that is designed to warm the floor surface rather than heat the whole room to an acceptable level.