Minimum each way price?

Assume a bookie wants to make a 10% RoI on the "shorties"

If he thinks a horse is a 6/4 shot he needs to be able to lay it at 2.25 to have an EV of 10% (2.13 for a 15% EV)

If he thinks a horse is an evens shot he needs to be able to lay it at 1.8 to have an EV of 10% (1.70 for a 15% EV)

If he thinks a horse is a 4/6 shot he needs to be able to lay it at 1.50 to have an EV of 10% (1.42 for a 15% EV)

If he thinks a horse is a 1/2 shot he needs to be able to lay it at 1.35 to have an EV of 10% (1.28 for a 15% EV)

If he thinks a horse is a 1/4 shot he needs to be able to lay it at 1.13 to have an EV of 10% (1.06 for a 15% EV)

Lets look at longer prices:

If he thinks a horse is a 10/1 shot he needs to be able to lay it at 9.90
to have an EV of 10% (9.35 for a 15% EV)

If he thinks a horse is a 20/1 shot he needs to be able to lay it at 18.90 to have an EV of 10% (17.85 for a 15% EV)

If he thinks a horse is a 50/1 shot he needs to be able to lay it at 45.90 to have an EV of 10% (43.35 for a 15% EV)

How many horses trading at 51 on Betfair are available at 45/1 in the ring?

Conversely how many horses trading at 1.25 are available at "only" 1.13 in the ring?

(I'm taking in general here, I'll bet there are isolated examples of both of the above)

Bookmakers definitely price more margin into higher priced horses (if by "margin" we are saying expected value from the bookmakers point of view). Theoretically they should not but in practice they do

Assuming all gamblers are rational & that bookmakers can lay in the same proportion as the horse contributes to their over round they should apply the same margin (where i define margin as EV from the bookies point of view) to short prices horses as longer priced horses, it maximises their EV.

There's a number of reasons they don't. One of the main ones (especially applicable to betting offices) is punters aversion to betting at short prices. I'm talking your average office punter here, not your hard nosed professional. I'd also assume that prices for horses with more liquidity are assumed to be more likely to be priced correctly. The less liquid prices are more likely to be inefficiently priced hence more margin.

Probably should add here, you actually can't price a 10% EV into a horse who "fair" price is 1.11 or less

The odds the bookmaker would have to lay at would imply (s)he gets a payout if the horse wins or loses

If p is the "fair" probability of a horse winning implied by the "fair" price then the (decimal) price, X say, the bookie needs to get 10% is:

-p*(X-1)+(1-p) = 0.1

as the bookie loses (X-1) with probability p
The bookie wins 1 with probability (1-p)

So the required decimal price is 0.9/p

This is <1 if p>0.9 i.e. the fair price is less than 1.1111111.....