It's funny how people quote prices for used Macs in their respective countries without even remotely considering the possibility that OP might be living in a country where computers either cost excessive amounts of money or are expensive compared to the population's average level of income. OP already stated that, and I quote,
"In my country these rams are pretty expensive..". This makes me believe that he or she might not be living in an affluent rich Western country like the UK, the US, or any other first world country but somewhere else instead.
So tell me: how exactly is telling him to get a 5-year newer iMac from a completely different country going to help?
@emrebzdag It's hard to say whether or not your workflow is going to benefit from more RAM. If Photoshop is slow because you're running out of RAM (check the Activity Monitor to find out whether that's the case) then yes, increasing your RAM by 50% is certainly going to improve performance dramatically. There is, however, no point in getting 2x 4GB because your iMac is not going to be any faster with these than 1x 4GB and 1x 2GB: it is not going to run in dual channel mode anyway.