Wales used to be very good for £5 pitches, away from the hot spots which had coastal views. However, inflation will have happened since then (particularly in water / waste water regulations) and in current times you really need to pre-book formal campsites.
Some places in the north:
If the OP is heading straight down from Llandudno to pick up the NCN8 around Porthmadog, Hafod y Llan National Trust site is first come first served and £9 per adult.
Many years ago I have stayed at Hafod Dywyll, in winter, was very basic (just a toilet, looks like they have showers in summer) and it would be an uphill slog to get there for one night if on foot/cycling, needs booking, £6 per adult. http://www.hafoddywyllcampsite.co.uk/rates
I’ve never stayed there but Cae Adda looks lovely and £10 per head: https://www.caeadda.com/camping
Merthyr Farm above Harlech is also a slog to get up to but has great views and cyclist rate (£10): http://www.merthyrfarm.co.uk/contact.html
There are probably plenty of others but you do need to plan and contact them before hand. I would say properly wild camping (pitch late, up at dawn) in forestry areas will be ok but yeah I suspect it won’t be tolerated in farming areas given the amount of litter and bad behaviour in the last year. I recon in the more central bits (after Machynlleth but before the Brecon Beacons) you’ll struggle for the more basic campsites (the type that allow fires).