you are worth what you can convince anyone you're worth. Fine art should carry a fine price and I wouldn't touch a 20x30 for $180. not to be a jerk, but I wouldn't sell a 20x30 for less than 3x that.
at $180 you're giving $60 to the gallery by the suggested math. I'd suggest that 40-50% is more common until you're a big name. either way, if we stick with the suggested numbers you're pulling a $60 "profit" out of a 20x30 image. it takes me roughly an hour to print, cut, spray and stretch a 20x30 print (obviously not all at once, but that's my best math at my most efficient, not counting retouching). I am absolutely unwilling to earn that little on a fine art print that big. Canvas is a pain in my rear and I want at least as much profit out of a canvas as I do a paper print. It initially seems like you're charging on some modified material's cost. I believe the biggest mistake a person can make is selling the paper/canvas. you're selling the image not the stuff.
May I suggest this exercise for any pricing discussions:
take what you want to make in a year and multiply it by 1.3 (that's bastardized math for the self employment taxes you'll have to pay)
write down all of your expenses (be realistic, include your equipment, maintenance, rent insurance, EVERYTHING that a real business pays for)
ask yourself if you want to retire, if so, include an appropriate amount for that
add the above numbers up and write the total down.
ask yourself, how many prints/wraps/whatever are you honestly likely to sell in a year.
take that number and divide it into the bigger number of what you need to gross to cover expenses and a salary that lets you eat more than ramen. this will give you your required average sale, from there you do the math on how you need to price things.
The above is a waaaay over generalized list, but the moral of the story is that every photographer should be aware of their business accounting and set their pricing starting there. Don't get too caught up on what other people charge. in every market there's someone who sells stuff for 5x what anyone else does and they are able to do so because they've convinced their audience they are worth it.