It would depend on what is on the books first.
If there is a sell x @ 100 order on the books (i.e. part of market depth) and you place a buy x @ 110 the trade will execute 100.
If there is a buy x @ 110 order on the books (i.e. part of market depth) and you place a sell x @ 100 the trade will execute 110.
Thanks DeathAndTaxes, that was the answer I was looking for. I understand it now, I think. Orders already on the market determine the (lowest, highest) current market prices, and an order placed later will be matched against those.