You rarely pay the resort fee ahead of time. Hotels.com, even MLife won't let you pay for everything ahead of time. The sites will show you the final price, but you are still going to settle up with the Hotel when you check in. Then, they usually throw a resort fee or $50-$100 on your card as a hold or for room charges/mini bar. I'd look at your detailed bill from Expedia and be sure they charged you for the resort fee. Your beef, if you have one, will be with them.
Vegas just raised their hotel taxes. I booked my room for March Madness and in the fine print, it told me I'd be paying more than I originally thought. Some sites break down the cost of the room, tax, and resort fee better than others. I know on Hotels.com, they highlight the price in bold and it says the cost of your room. On the next line down, it says when you arrive at the hotel, you will also pay: $xxx (usually the resort fee). Then, it will say the TOTAL cost of the stay = $xxxx. You have to read it carefully, Vegas always tries to make you forget about the resort fee & tax which can add up, especially on an extended stay.
Also, for the future, sign up for MLife, Harrah's Total Rewards, etc. and book through them or book direct with the hotel if they have a deal. It will always be cheaper than Expedia, Hotels.com, Kayak, Orbitz, etc. Not by a ton, but it helps to cushion the useless resort fees...and now parking fees for MGM properties.