The important thing to remember is that a tablet is a computer - it will do whatever your current machine can do, although it may use different software applications to do it.
So... ongoing costs.
First off, buy a case for your tablet. My iPad has a fancy leather one, because it travels with me more or less wherever I go, and I wanted something that looked nice, and would withstand being handled a lot.
Other than that, your ongoing costs will be limited to the amount of software you decide to buy. If you decide on an iPad you'll be able to get more or less everything you want at little or no cost.
What else? Nothing really - just enjoy mobile computing in an easy to carry format.