The entire thing comes down to market forces, the more people that back a specific horse the lower the cost will end up. This applies both in the high street bookies and on the betting exchanges.
Considering the regulations and laws with the US authorities and online gambling, your choices of sportsbooks could be smaller than for other people. You need to be certain you pick one where the depositing and withdrawal options are legitimate and are as easy as possible, so you don't get stuck in a situation where you can't withdraw your winnings or anything like that.
So next you want to check at the market. If most of the market is hovering around 4/1 or less for this occasion - then you should not bet. Read More In this article is a tough discipline to learn. You may feel that your bet will win by betting with this occasion but you shouldn't because you are not getting value.
Where value is always found in the choice of bets and the best odds are found each time, profits should be made. Any staking strategy, however, should allow for one of life's facts: that bets will lose. If the loses are limited by correct staking this won't deter from winning bets.
In contrast the traditional bookmaker, even though he now has an internet presence relies on a human being to make his market and therefore frequently exposes differentials in the market prices between himself and the more rapidly moving betting exchange. In short it will become possible to back the horse with a traditional bookmaker at a higher cost than the lay chances at the betting exchange.
By way of example, fractional odds of 1/1 which are exactly the same as decimal odds of 2.0 or American moneyline odds of -$100 (get a much better explanation of those here: Wikipedia - Fixed-odds gambling) give an implied probability of 50%.
If you wish to know your winning percentage in comparison to cost to predict the equation is as follows: cost to call divided by the money in the pot plus the cost to call. So from the above equation you'd have $25 / $275 that comes out to 9%. If you are chasing an open ended straight draw the numbers are on your side to making a good call. If it's an inside straight draw and you do not need to make another bet on the turn if you don't hit, then it's a solid bet as well.