It seems that a decent achromat is an important part of a good image acquisition in the 35mm adapter. It can also be the most expensive part if you use good stuff like Century Optics. I found Raynox DCR-250 for 60 bucks at B&H. Although it is not advertised as an achromat, it is one according to http://home.planet.nl/~heuv0283/achromats.html. It seems like a good alternative to Century Optics, which is $200+. Its also pretty versatile in terms of its mount (clip on). Anyone had any experience with it? Is it even in the same league as Century Optics?
Also, if anybody has experience with other reasonably priced macros, please share.

I don't know about the Raynox , but you can try the Hoya macro +10
http://www.bhphotovideo.com for 80 bucks , lots of people been using it with good results.

Can you point us to any threads where people discusses their success with the Hoya?

here are a few things on it.

People that been using it for their adapters :
http://www.dvinfo.net/conf/showthread.php?t=35954&highlight=hoya+macro
http://www.holyzoo.com/content/35mm/
http://www.enormousapparatus.com/35adapter.htm

you can see a macro comparaison here:
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/A95/Close-Up/Close-Up-Lenses.html

more digging...
http://www.dvinfo.net/conf/showthread.php?t=20408&page=9&highlight=hoya+macro
http://www.dvinfo.net/conf/showthread.php?t=26183&highlight=hoya+macro

Thank for the links Pete. Do you guys know if there is a mathematic formula to figure out minimum focusing distance using different macro lenses? I am using gs250 and its focusing distance is about 16inches. Would +8 macro(Raynox) be enough? Trial and error method can be quite expensive.

I think the idea is stronger is better, but the stronger the macro, the more barreling I believe you get. Go for the C.Optics or something that has a name brand.