How long is a piece of string?
There are many types of bike and many definitions of 'decent'. For example the Bazango has a fork that is very far top of the range but it is still better than entry level. On the other hand the brakes on it are the most basic hydraulic disc brakes and 'nice' brakes would be a bit more money. Some people however would insist on full suspension for MTBing, which will cost more money because you have a more complex frame & a rear shock to pay for. Others will say you need a dropper post.
Hence IMO the Bazango is MORE than decent, as this is £350 and quite nice enough. https://www.decathlon.co.uk/rockrider-s ... 79537.html
If you're a mug, and like your bike to say 'Trek' rather than 'Decathlon', you can pay £550 for a very similar bike. https://www.trekbikes.com/gb/en_GB/bike ... 7/p/17382/
MTB specs to look at:
(a) tyres - not particularly important as they can be changed
(b) rims - double wall aluminium, 32 hole is standard. wider rims are standard these days than previously
(c) hubs - anything Shimano branded at all is fine. Often this is not disclosed. But possibly a cheapo bike may have an off brand?
(d) fork - the standard entry level is Suntour XC. This goes XCT (worst), XCM (better), XCR (slightly better). The XCR exists in 'Coil' and 'Air' variants, while XCT & XCM are both coil only. Some people claim that XCT is unusably bad. I don't know if this is true, and may have been in the past. It is notable that XCT & XCM have roughly the same weight (around 2.8kg), while XCR is more like 2.3kg/2.4kg. I use XCR coil for thousands of miles & it's just fine. There is then Raidon, then Epixon, which are air only. You may find a 'Rockshox' fork, generally these are more expensive for the same performance than Suntour and are another one for the label snobs. There are various cheapo coil Rockshox fork to impress the unwary.
Other brands besides these two on cheap bikes may be a sign of very cheap rubbish.
(e) Brakes - Shimano hydraulic brakes are all good. Other brands of hydraulic brakes are probably ok. Other brakes may be a sign you have a cheap bike.
(f) Shifters - cheap bikes may have Shimano 'EZ Fire' shifters/brake levers. They are integrated into one unit & look clunky. They work just fine but are a pain for upgrading/repairing and best avoided in favour of seprate units. SRAM is another fine choice. Other brands than these two may be a sign of cut corners.
(g) Cassette - a cassette is a minimum, a freewheel is a sign of a supermarket 'BSO' basically. The number of speeds doesn't matter that much in all honesty, they all work fine.
(h) Derailleurs - the cheapest have 'Shimano Tourney', which says 'cheap', but actually works just fine. More expensive Shimano models may be better. Clutch-equipped derailleurs are an improvement off-road and a sign of a far-from-cheapest bike.
(i) Drivetrain - 1x is slightly lighter and fine if you drive to the MTB trail, and generally not on cheapest bikes, but not so good for road miles. Expensive or impossible to change down the line.
Road bikes are a bit less involved, as you don't have to fret about forks. Basically more money = lighter & better components, and if you avoid freewheel bikes then you will be ok. Any Shimano setup with STI levers works fine as a basis, more expensive models are not necessarily going to shift better if they are not kept well fettled.
The basic spec is thus Shimano Claris 8 speed with a front derailleur. These days you'll probably find a 'carbon' fork, as they're quite cheap from the far east, and reduce weight & road buzz (a bit).
In 'Trek' logo £600 https://www.trekbikes.com/gb/en_GB/bike ... -2/p/23522
Decathlon make one with a clone 'Microshift', which seems to be ok and is £350 https://www.decathlon.co.uk/triban-500- ... 79069.html
It's heavier than the Trek though.
If you want a 'gravel'/'adventure' bike you'll pay a substantial premium because you're paying for 'new' tech (which is nothing of the kind, just repackaged), and the manufacturers force you into a more expensive drivetrain, in this case a '1x' drivetrain which is more expensive than basic, proven 2x or 3x systems.
The same thing applies to road bikes with disc brakes, particularly hydraulic discs. These are cheap as chips to make but they want to milk the current trends by charging hundreds extra for something they knock out for £30 on mountain bikes.
Hence £1400 for a (admittedly otherwise nicer) hydraulic disc-equipped Trek road bike https://www.trekbikes.com/gb/en_GB/bike ... c/p/17352/
If you can make do with caliper brakes, or just ride your MTB when it's raining/whatever excuse it is you have for 'needing' discs, you will save lots of money. And wait five years and probably buy a new hydraulic disc road bike for £500.