You will have noticed that our choice of the best budget mountain bikes is mostly hardtails rather than full-suspension bikes. The reason for this is that if you are working to a tighter budget you will often find that brands make critical compromises to the spec of a bike in order to cover the additional costs of development, suspension system hardware, and shock.
The fact that most big manufacturers don't bother to risk their reputation on compromised low-cost suspension bikes should be a warning too. That's because most full-suspension rigs from 'bargain' brands have outdated geometry as well as poor quality forks and shocks – and will actually ride worse than a sorted hardtail for the same money.
That said, there are some brands, like the Marin and Vitus featured above, that manage to add front and rear suspension without sacrificing the overall performance of the bikes. Neither features a dropper post though, and components will generally be of a lower quality than a hardtail of the same price.