The consensus is that a cheap mattress is a false economy – spending, say, less than £300 for your daily-use mattress is almost certainly not going to ensure a good night's sleep, unless you are very young, very light or frequently sleeping somewhere else.
Or you will need to replace the mattress more often, perhaps as frequently as every 2 years. Spend a bit extra, say, around £500, and you should not need to change the mattresses for around 5 to 7 years.