I think so, based on a very small amount of research I did today.
I was looking at Marriott properties in Norfolk, VA, USA near the airport. I checked the rate at the Marriott site and then routed through TopCashBack to earn my credit. The rate was somehow $17 more expensive. A nearby property went up by $18 when TopCashBack was used.
These are the rates and properties: (Before TopCashBack)
The rates when using TopCashBack: (currently 5% earned or about $5 for these properties)
I need to do some more research over the weekend, but for now it looks like prices go up when using TCB.