To answer this question we need to know what sort of locks you have, and if you have a wooden door.
If you have a simple yale-type nightlatch with a round cylinder rose on the outside and a plain knob on the inside (very insecure)a replacement cylinder can be bought for a few pounds and fitted by anyone skilled with a screwdriver and pliers.
If you have a British Standard mortice deadlock (which you should have) then even less skill is needed, PROVIDED that you buy a replacement which is exactly the same size including the position of the keyhole (even a variation of 6mm may mean you need drill and chisel work). The newer the lock is the more likely you are to find an exact match. Look at the front of the mortice lock, it should have a kitemark which in tiny print will have a BS number and year (the year is important). Given this, and the make, which will also be on the front of the lock, I can give you a good idea of how easy it will be to match.
If you are paying someone to do it, the Best Buy mortice deadlock (per Which test) is the Union J-2134E at about £30 with three keys, available at any DIY shed or hardwarte store or locksmith.
The "E" at the end of the model number of many new locks mean they have an "E"xtended bolt, meaning the brass psrt that comes out to engage with the keep in the frame is longer, and usually thicker, than old older locks, so it is more resistant to levering and will engage well even if the door is fairly old and loose with a big gap.
If you have a plastic door, it will probably have an oval "Eurocylinder". These are easy to buy and fit but they vary in length and you have to get the exact same length again.
Cylinder locks, unless extremely expensive, are not as secure as mortice deadlocks.