Permit aircraft are 'generally' home built group A aircraft and all microlights.
However, your post brings up a valid point - it's not necessarily what you fly, but how you fly.
The training costs should now be roughly equal if you go down the NPPL route as the group A and microlight are type ratings of the same license (albeit with strange quirks and ambiguities). It seems strange that you can pay £100 to train on a microlight and roughly the same for a PA28 / C150 - anyway.
Flying microlights is far from cheap as you have to purchase your own aircraft - rentals are rare, require solo flight only and will generally be in a training aircraft that has a non useage window. This is hardly flexible and reality means that you will have to purchase a microlight to fly at the end of your training. We won't go down the road of the £20K + hotships, but lets look at an older and used Thruster TST for about £6,000 for example and look at what 5 years worth of flying might cost you.
5 years
Interest on £6,000 - £15,000
insurance - £4,000
hangarage - £2,000
Total non operating costs over five years £21,000.
Stats suggest that a typical PPL achieves 30 hours per year flying.
Direct flying costs for 150 hours (5 years) would approx. be:
maintenance £1,000
fuel £2,200
two stroke oil £400
Total approx operating cost £3,600 + non opearting cost (£21,000) = £24,600 / 150 hours = £164 / hour providing that you can sell the Thruster for the same £6,000 in five years time.
Even if you completely remove the 'interest' element because it doesn't suit your way of viewing the figures it still works out at £67/hour - NOT CHEAP
Expand these figures outwards with the costs incurred for 'hotship' micros costing £20K plus and you would have to wonder if these people did maths at school or just like wasting money.
Microlights and group A aircraft can be cheaper to operate if the pilot changes their attitude to things - for example very low cost and de-riggable flex wings save money on insurance, hangarage and of course on capital outlay. More expensive aircraft of both types can be flown cheaply via group and syndicate schemes and many, many of these exist whereby a lump sum followed by a monthly payment and a wet or dry cost per hour used means that all members of the group contribute to the upkeep of the aircraft in proportion to their useage.
Personally, I fly microlights quite cheaply because I am in an informal group of 3 who operate a CFM Shadow and we share the on going cost three ways. I am looking for a similar group A arrangement to make that equally affordable, but I don't object to the walk in/walk away cost of renting a PA28 Club aircraft at £85/hour which no doubt could be negitiated down a bit with guranteed useage.
Sorry about the length of the post
Cheers all