Ever missed your flight?
This might sound obvious but it is what it boils down to, it very much depends on the 'quality' of the carrier, and by that I mean service.
Some low-cost carriers will charge you a missed departure fee, and the difference in fare between your original flight and the one which you are transfering to (usually more expensive due to being last minute), however some low costs charge full fare even if you just miss check in.
Charter Airlines / Tour Operators will usually reprotect you onto their next flight for approx £100 per seat, or onto another charter airline that is in the 'mutual aid' agreement for around the same price.
With full service scheduled Airlines there are 22 (at my last count) different ticket classes (some which are basically the same) and those fares have different rules as to what you can and cant do with them. For instance J class would usually allow you to transfer onto the next flight FOC, and to make changes FOC in advance. Q class would be the equivalent of a low cost ticket, and no changes are permitted and is not transferable.
Basically, with a few exceptions it boils down to like pretty much everything in life, you get what you pay for. Low cost is great but by the very virtue of the business model you will not get anything for nothing if you pay £10 return, however if you purchase a Singapore Airlines Biz class ticket for £2500 return, then you will.