From On Nut BTS what's the fastest and least expensive way of getting to KM25 on the Bangna-Trad highway?

Minivan?

KM 25 is Bang Bor/Bang Sao Thong I think?

You can take the airport bus/minibus to Bang Phli Yai (25B) and from there it's not far by taxi or just get a cab all the way from On Nut (it shouldn't be more than 300B)

The best and easiest way to get to KM25 on the Bangna-Trad highway from Onnut is to take taxi straight there. It won't cost you much and will safe you more time going by other means of transport.

Cheapest way would be to take the #132 a/c bus (yellow)from On Nut to the HOME PRO/LOTUS in Ban Plee, change buses or take a van there for the final 5km.

You could also take a #38, 46, 48 or 132 bus from On Nut to CENTRAL-Bang Na and take a van from there, they pick-up just down the road past BIG-C.