The minimum odds you can bet on is 1/5. First 1/5 I can see on Betfair for that game is Man Utd HT DNB, so to lay that off, you'd need to bet on Saints to lead at HT, which is 11/2, meaning £9 will cover it (returns £58.50). You won't get a better price than that, as the fact it's DNB means you're getting more generous odds on the away win - the typical opposite price to 1/5 is 10/3.