The Duron 1.8 GHz sold at newegg and tigerdirect for $43 US or $68 CDN at tigerdirect.ca is more powerful than the Sempron 2300+, 2400+, is about the same as the Sempron 2500+ and can almost catch up to the Sempron 2600+. (It's an Athlon 2200+ with less cache.)
The Athlon XP 2700+ is $81 US at newegg and is one of the better deals because it beats those CPUs easilly but doesn't cost too much.
The reason you may not have gotten any answers is because you named a socket A processor than one for socket 754 or 939. The Athlons for socket As are known as Athlon XPs, not 64s.