I have a private plate on the zed, and was lucky enough to come across it just before getting the zed's predecessor. For a start, there's this bloke (ok, he's not real) and me, but there must be a lot of people with the same initials because all the best plates (e.g. prefix with one or two nice numbers) from the DVLA had gone.
My plate came from eBay attached to a car. Allowing for costs of travel, fixing the car's two punctures, selling the car again, transfer fees, etc, it came to about £250 in total. If I hadn't found that plate, I would have gone for something ageless meaning nothing to me. I'd have been happy with something like deka's above, as plates with an X won't be many people's initials and can be picked up not too expensively.
I have R3 and have since seen R4 and R8 come up one eBay, the first one asking too much, the second also attached to car. I sometimes wish I'd snapped up the R8 plate but it would've been a load of hassle and probably end up costing £500. eBay comes up with roughly one car a week with a number plate containing my initials, though few are any good.
My daily driver also has a personalised plate. I wouldn't personally have spent what it cost, but the first owner put it on and it's specific to the car and makes it distinctive.