"For me, Costa Rica is more expensive that living in the States (-60% housing, +50% car/gas, -50% medical, +70% computers/electronics, -30% utilities, +50% groceries, +50% entertainment, etc...) and negative on income (90% less taxes doesn't offset 50% less salary)."
As Adam said, it depends on YOUR lifestyle
In MY personal exprience groceries are about the same (with 300 USD I pretty much buy everything needed for a full month while in the States I have seen that 300 USD doesn't really go that far , of course it DEPENDS on what you buy)
Utilities (Excluding internet) most be like -70% of what it is in the US, 99.9% of houses here don't have heating equiptments so you don't spend money in heating oil. Water bill is about 6 dollars/month (in my case I have a bunch of computers running all the time and that makes my electric bill "high", that is I pay about 50 dollars /month), phones are cheaper also, I don't need "unlimited calls at 'x' time" , I barely go over the 60 minutes included in the minimum rate and that makes my phone bill around 8 dollars/month (don't think any cell phone provider has anything even CLOSE to that rate)
entertainment in my case is probably +80% , the other day I saw a dvd in Plaza del Sol, it was going for almost 35 dollars when I could get it at 10 dollars through amazon. If you like to watch dvds you better get a forwarding service that will send your mail down here from the states and buy them at amazon
Clothing is another thing that it can get REALLY expensive if you want quality clothes.
Medical can be (I supose) up to -80% if you need surgeries/medication....healthcare is cheap and good quality or affordable and high quality.
If I were single and owned my house I would say that making 900 USD/month would give me pretty much everything I need/want but being married ......well it requires quite a bunch more of $