1. On your engine's power output curve/diagram choose your normal operating rpm and notice the engine HP output.
2. Measure or guestimate the boat speed in still/flat water while operating at the rpm in #1 above.
3. With the dinghy attached and towed, increase and record the engine rpm to gain the boat speed in #2 .... defines the amount of work to overcome 'dinghy drag'.
4. Go back to your engine output chart/curve and note the difference in HP output between #1 & #4.
5. Then calculate (or measure) the fuel consumption between #1 and #4

Most boats of the approx. size we own have an 'immersion factor' (inches of increased displacement per 1000# of added weight), and most dinghies weigh approx. 100lb. ... calculating to an increased immersion factor of NIL resulting in NIL loss of boat speed or NIL increase of engine load/output when the dinghy is on davits.
Most inflatables being flat-towed by a boat will 'bleed off' about 8-10% of operational cruising HP from the 'mothership' ... if you translate that to gallons per hour, and do sometimes 'motor' for 'long distances' (such as a trip up and down the AICW), you'll quickly find that the towing cost will be quite near the price of added davits.