How much is the data costs.

Booklet seems to suggest 25p a day for unlimited usage, but website suggests 25p per 1 meg.

So if I estimate my monthly usage would be 500mb, would The charge be £7.50 (30days @ 25p) or £125 (500 meg at 25p per meg).

I used to be on a monthly contract, but rather be on PAYG .

up to 25MB it is 26p per day.
for over 25MB its 10p per MB plus 26p per day

The cheapest option by far is Three on PAYG at 1p per MB plus 150MB bonus just for topping up any amount or you can top up £10 and get 650MB plus minutes and texts or £15 for unlimited data plus mins and texts.

Could they make that anymore confusing and hard for people to work out

Are you on commission from Three?

No it's Giffgaff that pays users to spam, anybody else is just genuinely recommending,

What's to say one can't genuinely recommend GiffGaff?

I've never been a customer and get no benefit from recommending them, but have done to a few people in the past...

Recommending someone the cheapest and smartest option for their needs

Just the blind promotion regardless of what's best for the user. The best, the cheapest etc when often it's not and they are only promoting it for money a lot of the time.