another bike?

I'm obsessing again with the thought adding another bike to my gear. Currently I have a Redline 925 single speed and a Colnago Strada road bike. The Colnago gets most of the road time riding with the club during the summer season (maybe 4,000 km a year). The Redline gets a few training runs in, a few commutes to the office, and general joy riding with my wife and others (maybe 1,000 km a year). But things are getting a bit tedious with the single focus on road riding. So, I'm looking at cyclocross or mountain biking or fat biking (snow & sand bike). However, I can't decide what kind of riding I want to focus on. Cyclocross has alot of great events in the area, mountain bike trails are nearby, fat bikes offer the added benefit of riding year-round and extending to the winter season. Also, I can't come to grips with whether I deserve to indulge in another bike. I've got about $5,000 invested in two bikes already. So, I've decided to revert back to a strategy I used 7 years ago when I first started riding. I would "reward" myself for training hard by allowing myself to purchase gear every 500KM that I logged. I'm going to "pay" myself the same rate that I get from my employer for car mileage, which is $0.40 per KM. So, for a $2,500 bike, I'll need to log 6,250 KM. I'm going to allow myself credit for the 144 KM I've trained since Jan 1st and credit for the $700 I already have saved for sports gear (I saved mileage cheques from work). So, I need to put in about 4,400 KM's before I make up my mind. That probably means I'm not buying until next fall, unless this really motivates me. Let the challenge begin!