Ok you either have a Mac thats getting along in age or your looking to get into Mac's you go to nearest Apple Store and see the cheapest Mac they have is 1k and up, you don't have that kind of money ?, that's ok you don't need it.
We will use the 13 inch Macbook pro for example 2.5 core i5 4 gigs of ram and a 500 HD, its 1100.00 at Apple, its a nice piece of tech ......but it's got a standard hard drive , weak ram and it's just a dual core. Going out on a limb here but if your reading this then chances are you don't have a grand for a thunderbolt monitor so having a thunderbolt port really doesn't mean much to you.
A 2009-2010 13 inch pro in good condition can be had from 350-500, Solid state drives are dirt cheap these days and a 240 gig SSD is going to run you 100 buck, bumping the Mac to 8gb 1066 ram probably 70.00. Assuming you bought a 2009 for 350.00 add your drive for 100 and ram 70 you have a Mac faster then the base pro by a huuuuuuge margin and you only spent 520.00 or about 50% of buying new and you have a dedicated video card which the new one doesn't , thats a pretty good deal.
Aside from having to buy the Mac if you currently have a pro and are debating upgrading or buying new the same rules apply more ram and a solid state drive will give you great performance bumps for way less then buying new.....but those retina pro's are so nice , take some of the money you saved and buy a sweet monitor and plug your mac via mini port to it and use it to get near or better image quality , shop smart peep's newer and more expensive often isn't the way to go.