If you are willing to build your own, $500USD can make a decent computer able to max Minecraft. Make a post in the computer hardware section of the forums or have a mod move this to the correct section.
30 - 40 Frames isn't enough. 60 makes it feel smooth. However, if it works for you, just upgrade your personal computer. Add some RAM and a new graphics card. If needed a new processor. For graphics card, I would recommend a Geforce 400 series at around 30 - 40 frames. For RAM you should at least have 4 GIGS, but 6 to 8 would work very well.
I wanna have a steady 30 - 40 fps while running the shapeshifter mod and multiplayer.
I'm looking to spend as little money as possible.
What is the minimum price I'm looking at here?
Me?....... Lesbian?......pony please
Hardware section: http://www.minecraftforum.net/forum/208-hardware-discussion-support/
If you need them all:
PCPartPicker part list / Price breakdown by merchant / Benchmarks
CPU: AMD A10-5800K 3.8GHz Quad-Core Processor ($119.99 @ Microcenter)
Motherboard: ASRock FM2A75 Pro4 ATX FM2 Motherboard ($79.98 @ Outlet PC)
Memory: Patriot Viper 3 8GB (2 x 4GB) DDR3-1866 Memory ($47.98 @ Amazon)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ NCIX US)
Case: Cooler Master HAF 912 ATX Mid Tower Case ($49.99 @ Newegg)
Power Supply: Antec Basiq 350W ATX12V Power Supply ($28.80 @ Amazon)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer ($16.97 @ Outlet PC)
Operating System: Microsoft Windows 8 (OEM) (64-bit) ($89.98 @ Outlet PC)
Monitor: Asus VS228H-P 21.5" Monitor ($119.99 @ Newegg)
Keyboard: Logitech MK120 Wired Slim Keyboard w/Optical Mouse ($14.98 @ Outlet PC)
Total: $628.65
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-03-30 02:27 EDT-0400)
Of course there is room to lower some prices if you need them to be.