really does depend..... i spent quite a lot on my 20 gallon but thats because i love high tech expensive equipment like tunze skimmers and tunze ATO with 150 watt MH light.
One should realize it is possible to conserve funds... buy used equipment, little live rock, and a counter-current skimmer (these work best in my view and experience). I got my 175 watt halide, with ballast and slightly spent bulb, for less than $60. My skimmer, new, was about $30, plus $20 for an air pump. The sand was $30, and powerheads were about $80 in total. The rock I took from my 55 gallon when I took it down, and is about 3x more than an average 20 gallon would receive; this cost by far the most, probably somewhere around $400 though that does seem like a lot; which is why one needs to use caution and self-awareness when 'shopping' for a reef tank.
I got many of my corals for free, as small frags; overall on livestock I have spent around $150. So really it all depends on your choice of 'things'. Some luck can help too.
Well i currently have a 20 gallon with freshwater fish, but i do really like and prefer saltwater.So i was thinking my next project was going to be marine, but i know nothing about it at all! And learning it will be really hard ! So wanted to know how much it might cost me to set up/change my 20g into marine ??
it really does depend.... There are many variables. Read around this forum, other forums, other sites, stickies, books, LRC. Then come back and ask us when you have got the general idea.