I don't know if there's a quick answer to that. In one sense, a good (like the intel) SSD will almost always be faster than a 10K drive. How much do you benefit from that, depends on your machine/workload. Windows boot, app launch could benefit, games might not.
Then, throw in cost per MB, different things you could do (RAID 0 of 7200 rpm drives, raid of 10K drives, mix SSD boot with std storage) and it's a lot to think about.
newegg has the velociraptor 600GB for 279: http://www.newegg.com/Product/Product.aspx?Item=N82E16822136555&Tpk=velociraptor%20600gb.
The x25M 160gb (upcoming version?) http://www.newegg.com/Product/Product.aspx?Item=N82E16820167019&cm_re=x25-m-_-20-167-019-_-Product
is $449. A pair of 750gb WD black, 7200 rpm drives in raid 0 would run probably under $150.