Given that our UCAV (note not RPAS) will still require approximately 80-90% of the systems fitted to a modern FJ (the only bits missing are the pilot, cockpit and ejection seat) it is somewhat difficult to justify a 20:1 ratio in price!!
IIRC, for every pilot that goes to war there are another 2 at home - either for roulement or LCR in work-up, so that's a minimum of 3 pilots per airframe. Also assume a reasonable airframe life - since it won't need to support routine day-to-day training flying - so over it's life that might equate to 9-12 pilots, all of whom need training, feeding, paying and a pension. Let's assume a total of £10M per head over a career. That's £90-120M in savings per airframe to offset against the purchase price.
IIRC, then every Lb of primary structure gives a saving of 20Lb in support structure, fuel etc in an airframe. Strip out the pilot and all the support equipment such as seat, OBOGS, cockpit etc, then the airframe can be half the size and much stealthier, or with twice the endurance - so with a CAP at 300nm from base it can half the number of airframes required to support it.
If the basic concept/equipment works, then the savings are certainly there.