www.tcwo.com has Duron processors ranging from $59 for a 600Mhz to $104 for a 800Mhz. To put it simply, Duron is to an Athlon as what a Celeron is to a Pentium III. It's fast and cheap, and performs almost as well as it's more expensive counterpart (and it will smoke a K6-3). I have used the Soyo K7VIA for Athlons, Soyo 6BA+100 for Pentiums, and soon to use the socket a Soyo K7VTA for Athlons/Durons. However, this board is around $108.00, making the total price higher than the range you have specified. The extra money will go a loooooooooooooooooong way though, and upgrading will be easier in the future with a newer generation board/processor. You can find the board at
www.mwave.com, as well as many other socket a boards.