I've learned a bunch from the folks at Apexhighoutputalternators.com. There's a guy named Nicholas Starks who has assisted me and is always helpful. Taught me about the pully size on my first purchase and why I needed a different size belt, hairpin phase, etc. . Their pricing is inline with what you posted here and my experinces with them are nothing short of good old school customer service. These are drop in replacments too. They even advised me on the part number for a belt even though they don't sell them (or at least he didn't try to push one on me if they do).
Send him a question on what you're looking for and what you are going to use it for, he's never steared me worng. When I qasked for a quote on my Mazda 3, they sent this back.
250 amp small case 6 phase hairpin $319
(120-140 amps at idle)
320 amp small case 6 phase hairpin $379
(160-180 amps at idle)
350 amp small case 6 phase hairpin $419
(200 amps at idle
Powdercoating is $35 additional cost in any color, shipping is free in the United States.
Never hurts to inquire.