A free bet is a token, not money. It sits in an account, it can be used once, and when it is used the stake is normally not returned with the winnings. Understanding that one detail explains most of what follows.
Stake not returned
Back a selection at 6.0 with £20 of your own money and a win returns £120: your £20 back, plus £100 profit.
Do the same with a £20 stake-not-returned free bet and a win returns £100. The stake is not yours, so it is not given back.
This is why a £20 free bet is not worth £20, and why the sensible question is not "how much is the free bet" but "how much of it can I keep".
The terms that change the value
Minimum odds. Most free bets require the qualifying bet to be placed at or above a set price. Below it, nothing is triggered.
Expiry. Commonly seven days, sometimes less. An unused free bet is worth nothing at all.
Split bets. A £20 free bet issued as four £5 bets is not the same offer. Each piece has to be placed separately, and the extraction on small stakes is proportionally the same but the effort is four times over.
Eligible markets. Some free bets are restricted to particular sports, markets or bet types, which limits how well you can match the price.
Settlement rules. Void selections, non-runners and each-way terms all interact with free bet rules in ways that are specific to the bookmaker.
The realistic range
With a well-matched price and low commission, a stake-not-returned free bet typically retains somewhere in the region of 70–80% of its face value. Poorly matched, that falls quickly.
Rather than take those figures on trust, put your own numbers into the calculator — the retention percentage it prints is computed from the odds you enter, not from an assumption about a typical offer.
Nothing here is a recommendation
This site does not list offers, rank bookmakers or tell you which to use. It does arithmetic on numbers you supply. Whether any particular offer is worth your time is a judgement about terms, and the terms are on the bookmaker's page, not this one.