Grand National Avg odds on laying a horse?

Sorry, pressed return before I could include my message...
Also not sure if the title makes sense to anyone but me so apologies for that to...
I've only recently gotten into horse racing so this will be the first Grand National I'm interested in on Betfair.
I'm not intending to pick the winner on Saturday but one of the losers. So my questions are about the odds for laying horses.
Do the knowledgable and experienced on here know if is there an average number to lay at least half of the horses before the race. Say 20s to 40s? Or is it much like the regular horse racing odds where there will be a few favourites with laying odds under 10 and the rest esculating up to higher numbers for the outsiders?