Obviously, all depends on your mains. List those so we all know what you are up against.
most center speakers are just main speakers turned on its side, the aerial cc3 center was 1 of the very few that was actually designed to be a center, I owned 1, after selling it, I went through several centers, aerial cc3b, aerial cc5, definitve, etc, I recently bought a used aerial cc3, and won't make the mistake of selling it again, the aerial may be a little more then your budget, but it is the best, good luck
You have some homework cut out for you. The first is decent and $300. You may have to raise the bar slighly as very few dedicated center channel speakers show up on AudiogoN. Ya gits whatcha pay fo' especially at the mid-price ranges, (diminishing returns factor in at the extreme high price end). Second is time and timbre matching, and don't kid yourself, it is a huge factor as the sum of your parts become greater than the whole as an integrated speaker array. Your last post mentioned the Maggies - extremely fast with with many timbral nuances that might not even mate well with an elecrostat like ML's (a guess - they may), but certainly not a dynamic speaker or a horn. Try reposting your question with your mains (and sub?) mentioned an see what kind of responses you get.
Get a 4 or 5 inch dia. Helix coax car speaker (you may have to buy a pair, but so what? They only cost $120 per pair.) They sound far better than any of the usual small speakers you'll find in a typical upper-bracket Hi-Fi store. Put it in a little box with some fiberglas padding. I know that you've never heard of Helix - I just found the brand myself while looking for superb-sounding rear-channel speakers.
You can get them through "The Autophile" website. e-mail the distributor: he is very helpful.
Prior to my current center-channel speaker (Hales Revelation Center) I was using an Infinity SM-Video made in the late '90's paired with Infinity SM65 studio monitors. It had dual 6.5" woofs which flanked a single 1" polycell tweet in a fairly big ported box. While certainly not as refined as the Hales, it definately delivered crystal-clear dialog. It's been discontinued but could probably be found used for well under $300(original retail was around $350). NHT has some nice offerings in your price range as well, but I don't know if either would match your current speakers in timbre, etc.
Unlike traditional upright speakers, many horizontal oriented center channel speakers suffer from moderate to severe off-axis lobing which will muddy up the midrange. That's one reason they have earned a bad reputation for poor dialog intelligability. The frequency response and tonal quality can really suffer in as little as 10 or 15 degrees off-axis with some speakers. The more advanced designs may utilize a midrange driver and/or crossover tweaks, etc. to help minimize the lobing. You may have to invest a little more than your budget to get to where you want to be..