I fly both.
Learning to fly anything is expensive. The economics of running a flying school are not hard to work out, and when you do, you'll see why lessons are not that much more expensive for group A than microlights.
However, you do need a little over half the hours - at least in theory - to get a microlight PPL than a "group A" PPL, so when you compare 25 hours at £120/hr that's a nominal £3k, compared to a nominal £6,300 for 45 hours at £140/hr.
Once you have a licence, the running costs are similarly different. Just taking a quick look on AFORS, the first page of each category....
Cheapest 3-axis microlight:X'Air 582 at £5,200
Cheapest flexwing: Pegasus XL-R £1,800
Cheapest "group A":PA28-140 at £12,000
All perfectly good aeroplanes I've flown, but I'd struggle to explain why any one of them was more fun than the others. Roughly, I'd expect the Cherokee to cost around £90/hr, the X'Air around £45/hr and the XL around £35/hr all in.
So, in total expenditure terms, a money-saving microlight pilot can probably pay in the order of half the price of a similar "group A" pilot.
You can, needless to say, pay a great deal more money in any of the classes, you can also fly cheaper, particularly through syndicate membership, or the more basic LAA aeroplanes which are microlights in all but name.
But basically, expect the cost of "group A" overall to be about double the price to get your licence, and to subsequently fly per hour, as microlighting.
Speaking as somebody who flies both, I like group A for the ability to put people in the back seat, and to fly night and IMC - but for fun the microlights is at-least as much fun, possibly a bit more on the right day.
But - train in what you actually want to fly - learning one thing because you actually want to fly something else and plan to convert is never good economy.
G