The percentage spread they add on top of market spread is a published fixed percentage - and can thus be calculated in advance of requesting a quote - (eg, Cantor add 0.6% of share price, on top of current market spread, for a near quarter bet, and 0.8% on a far quarter bet), so they are not going to get away with quoting an excessive spread without being challenged.
The
position of the quote (ie where it is centred in relation to where current market price now is) is something they will be forever shifting, in line with future expectations for each stock. That is normal. If the stock price is regularly climbing fast, the quote will likely be further ahead of current price than if the stock is climbing less fast, and will likewise be skewed to the downside when the stock is falling if such falls look like continuing in a way that shows in the futures market for that stock or its sector.
Again, this is normal.
The
interest you refer to is part of the cost of a rolling bet (and doesn't show as a seperate item in a quarterly bet where it is contained within that added-spread percentage) and is nothing to do with how far away the quote is from the current market. They are free to position the quote wherever they wish and free to move it wherever they wish, whenever. The judgment they exercise in doing so will likely include logarithmic formulae relating to both the relevant futures market and to the weight of betting. If they wish to also incorporate a degree of craftiness specific to a prticular bet, and have the time to do so, they are free to do that too
I don't find it a problem. It's a competitive field; the ones who mess people about the most lose custom to those who don't.
PP
PS: It helps to remember that they are bookies, not brokers