Clients who ask you for it are the ones who know what they're doing, so it's worth giving them what they want. It's not legally mandatory for a generic freelancer, but if you work with serious companies they'll almost certainly ask for it before signing anything. Same thing happened to me at the start - I thought it was optional, then I had to scramble to sort it out.
As for prices, the thing is they vary quite a bit depending on what amount you insure (the coverage limit) and how much you invoice annually. If your portfolio is small and the projects aren't that important, you'll find fairly cheap policies, maybe a few hundred euros a year. Online agencies like Zurich, Generali, Heidrick & Struggles and the like let you get quotes in two minutes, as long as you have the info handy. Try at least three different places because the price difference for the same coverage is really noticeable.
The other thing is a lot of clients will be satisfied if you show them decent documentation, so if getting it is still a burden for you, you can always be upfront and say you're in the process of getting it. Works more often with small companies than you'd think.