all the port multipliers I've seen cots about $60, that is $12.50 per port plus the Sata card, which is going to add ~$2.50 per port.. You can get PCI four port sata 2 software cards for about $40 which is only $10 per port, and just stick as many of them as you can fit on your board. If you are running a separate media server like I am, just run it on an old system. Mine is an nForce 3 motherboard with 6 PCI slots, that would allow for 24 drives. I've personally streamed a pair of 1080P movies off a single PCI raid card running windows integrated software raid 5 with no problems or stutters at all.
There are a several intel 4 series motherboards for socket 775 that would allow you to run dual or quad core cpus with more than enough DDR2 and still have 5 PCI slots, that's 20 drives, plus they have 6 on board, subtract one for boot drive and one for optical and you've got 24 drives, drop in relatively well drives, that'll give you 48TB max, that should last a VERY long time. If you need more space than that you really need to up your budget by a couple grand to be honest.
Or, if you don't mind stepping back in time a small bit, you can grad a supermicro board that has four PCI-X slots and supermicro raid cards with 8 SATA 2 each, and still have 6 on board SATA 2 ports, a PCIe x4 and PCIe x8 slots for more raid cards later on, you could easily get 40+ SATA2 ports out of that board with raid cards.