I know this is very much "how long is a piece of string", but I'll have a stab at someballpark figures
In terms of size and complexity, you can bracket aircraft roughly in the following five levels:
(1) microlight / homebuilt VLA
(2) Certified VLA, gliders & motorgliders, uncertified part 23
(3) Certified part 23, part 27
(4) Parts 25 and part 29, military transport
(5) Military combat
Each time you go up a level, I'd reckon on very-roughly adding another zero to any cost, whether it's design, build, purchase, flight test or certification.
At level 5, Typhoon is pretty well documented at £3.3bn + £30m per aircraft.
I've worked at just about every level of these, which probably makes me fairly unique - although not entirely as I can think of a few other people who also have. Amazing I'm still sane really. Wibble.
So, drawing a line between these and tweaking a bit for personal experience I'd say development costs are in the order of...
(1) microlight / homebuilt VLA: £100-250k ish
(2) Certified VLA, gliders & motorgliders, uncertified part 23- £250k-£2.5m (this is the bracket with the greatest range)
(3) Certified part 23, part 27 - 25m - ish
(4) Parts 25 and part 29, military transport -£250m ish
(5) Military combat - £2.5bn - ish
For the cost of a flight test programme, very very roughly again, at each level I'd say that level is about the number of aeroplanes you'll need on the programme - so 1 for a microlight and 5 for a combat aeroplane; reckon on the test programme needing that many dedicated aeroplanes - and the cost of the programme being about the cost of the aeroplanes plus the same again. So, if a microlight costs £20k, reckon on a £40k test programme cost, and if a typhoon costs £30m, reckon on about £300m for the test programme.
The total certification programme cost is probably in the order of the test programme plus another 50-100% depending upon the levels of novelty in the technology.
There is a chap called Jim Scanlan at Southampton University in England (I'm not him, but I've worked with him) who is a professor of aerospace project costs - if this is a serious professional question, you could do worse than put some consultancy his way, or if it's an academic question, look up some of his published papers.
G