Risk v Reward
You'll often see that a high-quality horse race where the favourite is trading at 6 or higher will trade far less than a lower class race with a favourite priced at 1.85 in a small field. There are two reasons behind this - how Betfair calculate the matched volume in a market (double the backer's stake rather than counting the risk on both sides) but also because the average punter is far more prepared to lay a selection at a short price rather than a higher one.
At 1.85, the backer is betting £20 to win £17, the layer stands to win more than his/her adversary.
At 6.2, the backer is betting £20 to win £104, so the layer needs to have significantly more funds in his/her account to match that bet.
Novice punters on Betfair can be excused for thinking that laying a horse at 50 sounds like easy money - but it's a lot more risky than that. Studies by the Racing Post have shown that on British racing markets in particular, the closing market on Betfair is exceptionally accurate. Horses at even money (Betfair odds of 2) will win very close to 50% of races. Horses priced at 50 will win their share as well, very close to 2%. This means in the long run, say over 1000 races, the 'no-chance longshot' priced at 50 will win about 20 times. Horses and golfers especially when in-play, have been known to win after being matched at 1000! Risking £49 to win just £1 requires a lot of patience and nerves of steel. You might win 98 times in a row, but in the long-term, it will balance out.
Look at it this way - if you are laying at 50, you are backing the rest of the field at 1.02 - would you consider that as a smart bet?The shorter the price you lay, the more you stand to win. This also explains why bookmakers lay favourites for a living - they might win more often, but the damage is far less than taking a stand against a longshot on his lucky day.