Allan - Not to be argumentative, but a simple cost analysis can help you determine if the equipment will pay for itself, and how long it may take.
Let's say you determine that a drum sander will take 3 hrs per day, 3 times a week, and your shop rate is 80 per man hour. Lets say it takes 2 men to run the slabs. That's 72 hrs a month, at $80.00 per hour, or $5,760 per month of work (excluding belts, maintenance and electricity - that should be in the shop rate anyway).
Now lets say the wide belt with a rotary converter will do the work in half the time. This is simple to see that you can still charge $5760 for the work, but your costs are half what they would be for the drum sander. That generates $2800 per month in clear profit used to pay for the converter and wide belt.
That is, the converter will pay for itself in one month. That is an excellent ROI - most shops think about a ROI in terms of years, with 5 being the maximum in most cases.
Even if the wide belt / RPC will save you one hour per week, you will recoup over #00 per month. Then the converter will pay for itself (and install, wiring, etc) in less than a year. It is still a bargain.
None of the above takes into account that drum sanders are hobby equipment that cannot stand the rigors of industrial life and are designed as a stepping block to wide belts for those that are unfamiliar with the real thing.
After you pay for that RPC and wide belt - in short time, you will bag all the additional profit from that point out, and use that machine to upgrade other equipment, pay employees living wages, go to Hawaii, or whatever.