All the big auction houses have websites where recent auctions are listed, and you could contact them for a valuation.

She is probably represented by a gallery in NYC, that sells current and past works. Here is a website that has hammer prices but you have to sign up. It may be free I didnt read the details.

Here is a link that might be useful: ArtPrice

It may be Gagosian Gallery. I found one price of $6500 for a print in 2007, but it was a self portrait from the 70s. I don't imagine the non-portraits command as much.

I found the photo online. It's called Untitled (Queen of Hearts) and last sold for around $1400.