The bad news is that there's no way to accurately predict exactly how much power a particular transmission is going to suck from your engine. There are simply too many variables, including transmission efficiency, gearing, heat and even the type of fluid used for a particular application. However, there are a few rules of thumb that apply to most scenarios -- in particular, regarding manual vs. automatic transmissions and whether or not the automatic transmission uses a lock-up torque converter. However, a little bit of homework can get you fairly close to quantifying how much power you're losing through your vehicle's transmission.
Calculate wheel horsepower, given the known crankshaft horsepower and transmission type. Automatic transmissions and the associated drivetrain usually consume about 20 per cent of the engine's power, and a manual transmission takes about 16 per cent of the power. Multiply your known crankshaft horsepower by 0.20 for an automatic or by 0.16 for a manual, then subtract that from the known crankshaft horsepower to derive wheel horsepower.
Calculate crankshaft horsepower, given the known wheel horsepower and transmission type. Multiply wheel horsepower by 1.25 for an automatic or 1.193 for a manual transmission. Double-checking our work from step one, we'll calculate horsepower loss from a 300 crankshaft horsepower engine through a manual and an auto: Wheel horsepower should be 240 for the auto (300 x 0.20 = 60; and 300 - 60 = 240) and 252 for the manual. Working backward, the auto measurement of 300 horsepower (240 x 1.25 = 300) and the manual measurement of 300.63 (252 x 1.193 = 300.63) are close to dead-on.
Recalculate for an 18-percent loss if your car uses a lock-up torque converter. Even the best non-lock-up torque converter never achieves full lock; the lock-up converter clutches positively lock the engine and transmission together, resulting in a bump in efficiency. Multiply crankshaft horsepower by 0.18 or wheel horsepower by 1.219 to derive the measurements from each other. Using the 300 horsepower engine mentioned above, we find that a lock-up torque converter reduces wheel horsepower to about 246 (300 x 0.18 = 246); multiply that wheel horsepower by 1.219 and you get 299.87 crankshaft horsepower.
Bear in mind that these are fairly general guidelines intended to cover most road-going cars and trucks; a very large transmission, regardless of the type, is going to take more power to turn than very small one. If a particular transmission requires 50 horsepower to turn, it will require that same 50 horsepower whether its bolted to a 100 horsepower engine or a 500 horsepower engine. So, consider the application before espousing any estimation as gospel.