There are some minor modifiers to consider here. If the amp is near mint I would assume perfect structural condition, no mods (some people mod these using the reverb circuit as a basis for the extra gain stage without having to install yet another tube), original caps and Drake transformers, near perfect tolex, grille cloth, logo and silkscreening cosmetics, pots and jacks that are original and not noisy, missing or loose, a crack free non-warped vent cover (these amps tend to have warped and cracked plastic vent covers) and corners (these are plastic and crack or break without much force typically), an intact handle and end caps, reverb tank that still works, an intact DI knob (common for these to have been broken off), feet that are intact and still spring, a good two button footswitch and a set of good tubes. It would depend also on 2205 or 2210. I would say that a 2205 in that condition could go as high as $850. The 2210 might fetch $1100. This all depends upon the immediate demand for the amp and what the buyer is willing to spend on such a specimen. Some people don't know what they have. Though some of the criteria above is more show than go, depending upon your intention, your willingness to pay more or less can and will change.
These amps aren't your typical Marshall fare which is why I am sure that you are GAS'ing for one. They were a great highgain design that Marshall gave up on in lieu of the Dual Reverb heads of the 900's (mistake). The 800 channel switchers had a much better RAW Marshall sound. The clean isn't real clean but in a Marshall who plays clean? The reverb is a nice touch but isn't formidable. The original footswitches sucked for these amps. The replacement ones are much better. You can even find them that say Channel and Reverb. Marshall footswitches of the 1/4" stereo plug type are all the same. The footswitch takes you out of the "Boost" channel and also turns the reverb on and off. Without the footswitch you are in Boost by default. This is not so bad but you don't have full use of the amp either. Again some mod the reverb channel into a footswitchable highgain boost circuit. I have the schematic here somewhere if you want it and it is available if you look hard enough online. It takes the amp a little further without having to have a boost pedal because it is then built in and it is tube at that. There is also a tube buffered effects loop that I am sure can be modded into another gain stage or boost effect. I just haven't seen the schematic for that yet. There also isn't a footswitch for the effects loop either. I assume that the DI could be modified into an additional Master too but then again as often as the knob breaks off I would be careful going there. Really this amp was the cats meow for early highgain Marshall style. It won't sound like a Mesa or even try. It doesn't need to. It sits well in the mix due to its voicing and it can be pretty **** brutal. I would take one hands down any day of the week over a 2203/2204. Oh wait, I already did that before.
Mine is in fair condition with a few snags in the tolex with a dime sized repair on the front edge at the top. My DI is cracked off (I don't use it nor care to). The vent has the tips of the corners cracked off but the screen is intact. All my knobs work well and are quiet (again I don't care about the DI). I have a Randall replacement footswitch because I couldn't find a Marshall one when I needed it. There is a little chip out of one side of one of the handle end caps. Sounds like a trooper huh? It rocks so hard you better check your sister's socks. So cosmetics aren't everything. BTW mine is a 1988 with original Drakes and caps. I have Siemens EL34's in it with a hodge podge mix of 12ax7's. I keep that old mixed set in there to be able to swap around a little if I want different flavor. The Chinese makes it livelier in V1 while the Russian makes it darker. There is even an RCA in there just for kicks. The others I believe are an EH and another Chinese. I haven't run Mullards in it but then again why would I? This thing is all about being brutal. I can make it icepickier with the Chinese or mellow it out some with the RCA. I tend to run the Russian or Chinese in V1 though. This is not your mother's amp by any means...