Must be a temporary glitch.... But also possibly not... Could be an introductory offer.
Listed as 0.79 on UK store but between £4 and £6 if you try all the other foreign regions.
(Yes i checked as i thought 79p might be expensive haha)
Worth noting that the less add-ons for 10,000 chips are 79p and for 27,500 is £2.39. The next one up for 135,000 is £7.99 which really puts this deal in perspective.
Turns out its a nice cheap boost for players wanting some extra starting chips for this great free to download game. Invest quick as mistakes are usually rectified quite soon