The price difference is about ~$36 or a bit more than 55% greater. It's a 40% difference to the X2 550.
Athlon II X2 250 $63 / Athlon II X2 255 $68 / Phenom II X2 550 $88 / Phenom II X2 555 $99
Performance comparison of 3.1Ghz models Athlon II X2 250 vs Phenom II X2 550
Just on price vs performance alone the Athlon II X2 250 is a better value.
Still, the Phenom II X2 550 is only $25 more.
If you're interested in overclocking and you have a decent motherboard that is OC friendly I'd suggest the Athlon II X2 250 and a decent $25 CPU cooler. Sunbeam Core Contact Freezer $25 a/r and free shipping.
To me its worth the extra money for the unlocked multiplier and extra cache. If you don't care about the multiplier, then you are wasting your money on any black edition processor, because they are priced higher than the locked CPU's at the same frequency.