It all comes down to probability. Actual probability, not implied. Just because it's $51 on the tote doesn't mean it's got a 50:1 chance? It's real chance of winning might be 500:1, so if you can lay it for $250 you're getting unders.

You need to map all this out, find out where the edge is. I've done over 6 years worth for all Betfair prices, for all ranks and six different 1st fave categories. There is profit to be made there, but we're happy that backers think there's a gold mine in backing longshots. Keep it up