wolf68k

Cheaper price? In that case start with PriceWatch.com, they don't sell anything they only show you the lowest prices and let you pick the product and then link you to the online vendor.
You can also use Froogle, but you have to know exactly what you're looking for since ther are no catagories like PriceWatch.
There are a few other sites much like PriceWatch that will show you other vendors but the prices usually any cheaper or atleast not by much.

I found, via PriceWatch, a P4 3.0Ghz on a ASUS mobo for around $237 and another for around $250.
Now the first one (the seocnd one down on the right side of the page) includes a case which you can change for a few extra dollars or get rid of completely and save $30...however it's only uses AGP 4x, second don't include a case at all but it uses an AGP 8x which is I think the second is the better deal between the 2. The upside to this, if you as me, is that's it's a name brand mobo that is well known to be good...but it will cost you a bit more than the one you listed before which I still can't tell who made it.
There were others for less listed at PriceWatch, but they were mostly ECS and PC Chips which are cheap as well and you really get what you pay for. There was also AS Rock mobos listed, but I've never heard of them.

It's also possible to buy the CPU and mobo from different vendors and save some money that way and give you more choices of the make and model of mobo you get.