I keep it in a de-rigged hangar, which costs me £25/month. The biggest pain behind running a flexwing is rigging and derigging times. On a good day it takes about 45 minutes to rig and 30 minutes to derig and put it away - my normal solution to this is if a high is coming in to rig at the start and leave it tied-down for a couple of days until the weather deteriorates then derig and put it back in the hangar again. The flip side of this is the ability to stick the trailer behind your car and take it on holiday with you.
Flexwings are pretty benign things to fly once you've got the hang of them (took me about 10 hours to pass the differences training and another 20 to start to feel genuinely comfortable), you do need to be careful of hypothermia in winter flying (I solved this with a biker-type electric thermal jerkin), but other than that will fly in most conditions you'd fly a light aircraft. (I do admit to working up a bit of a sweat even now in more than 10kn of crosswind).
2-strokes aren't as bad as made out, what they are is intolerant of not enough (or too much) care and attention. My engine has just died at the tender age of 17 years / 500 hours from age-related problems and a replacement cost me about £600 second hand which isn't too bad. In general however they're only slightly less reliable than more traditional engines and are usually fairly sporting and give you plenty of warning if they're going to fail - I've had a couple of failures on the ground, one after a precautionary landing, but none in the air.
Routine work on a Rotax 2-stroke basically comes down to about ½-1 days work every 50 flying hours, which isn't too arduous. Parts are cheap - a set of plugs every 25 hours (2 or 4 depending upon engine, at £1.80 per plug), £5 worth of gearbox oil annually and a similar value of air-filter cleaner and oil each year.
The biggest expense is fuel and 2-stroke oil, which I suppose I use about £12/hr worth of in total, or roughly half my total flying cost in it.
One other gotcha with flexwings is the sail, which is worth usually more than the engine. Before buying one, it's vital you get a BMAA inspector to check it over and confirm that it's got plenty of life left in it - keeping it de-rigged in the bag helps here because the main killer is UV.
If you're looking for a cheap flexwing, I'd go for a Raven (what I fly) a flash 1 or a Pegasus Q, all cheap, reasonable performers with a good safety record and plenty of support available.
P
(Photograph gratuitously linked to from a microlight manufacturer's website because I liked it)