It should be a strictly business decision, so let's see. The most expensive painting on your site is $400. Double that and say you show paintings there for $800. On each sale your 70% would be $560. So before making a dime, you would have to sell seven paintings just to pay off their "representation fee"-- which would go even higher when they try to convince you to buy some actual promotion.
Galleries are supposed to pay the artist, not vice versa. IMO Vanity galleies like this prey on the desperate, vulnerable or ignorant, and working with one is like announcing to the world (and to other galleries!) that this is what you are. If I sound harsh, it's because I don't like predators.
That said, I know someone in Sacramento who once showed with Agora and claims he sold work. However, I don't know whether or not that means he ever saw any actual profit, and as far as I can tell, being able to say he was a "New York artist" in that venue did nothing to advance his struggling career.
One more thought: When most reputable galleries are flooded with requests from artists seeking representation, why is this one soliciting unknown artists?