On TRD's website they list a TRD oil cap for either like $53 or $85, on ebay there are ones that look just like them for $12 is there any actual purpose for them other than dress up? Is there any real difference besides one is a knock-off?

purpose = make you spend money

the stock one looks just fine

I would be leary of knock-offs. I had a chepaie one I picked up for $10 ... and when the engine got hot and the cap "expanded" it was impossible to take it off. I hadto wait for the engine to cool.

Oil caps are just dress up items.
You can a real TRD oil cap for less than $50 here:
http://partznet.com/toyota/mr2/TRD%20Parts/trdmr2spyderoilfillercaps.html

theres better things to spend the money on...