You're going to get a lot of different responses here because of the variation of incline angles people use.
Ideally, an incline should not be set above 30 to 35 degrees from horizontal, if you are truely going for a chest workout. If this is so, your weight used on an incline bench will be close (maybe slightly less) than a bench. However, if the incline is greater, the weight will definately drop, as you start to use more shoulder and lower lats to get the weight up. The incline benches at my gym are at about 45 to 50 degrees - typical for most gyms, and I truely hate them. I end up with more of a shoulder and lower lat workout than a chest, so now I only incline bench on the adjustable benches, as I can get them lower. This means using DB instead of BB, so I use that for deep stretch chest movements, then back to the bench for compound.