When starting a garden off, you often need to dig in compost. Ignoring the point that it may not be best to bring in compost from off site to improve your soil, how does one determine which of the composts are the best for your garden? I often see cheap composts at half the price of the more expensive ones, and I always go for the cheaper.
It's impossible to say whether a compost is 'good' or not without knowing what type of composts you're looking at - it's certainly a question more often asked about potting composts rather than compost generally.
The noun 'compost' on its own is confusing - there are two main forms of composts; first, potting compost of varying types for various purposes (multi purpose, seed and cutting, loam based, ericaceous, cacti for instance) and second, soil conditioning composts, of organic origin and meant to be spread on the soil, and that type of compost would include things like your own garden compost, animal manure composts, leaf moulds, spent mushroom compost and the like. It's safe to assume that commercially produced potting composts are sterile, or pathogen and weed free. As to the quality of the potting compost, that can vary enormously, depending on the brand, regardless of any description made about quality on the bag, and often, experience in use is the only way to decide which is good and which isn't. In the UK, we are lucky enough to have John Innes composts available - these are loam based, and are considered the best, though somewhat heavy once wetted.
Soil conditioning composts are not usually sterile, which is why they're good for the soil - which one is 'good' or not is again hard to say - if an outlet is selling 'organic composted animal manure' for instance, then that's what the bag should contain.
In the circumstances you describe, you'll be looking to choose a suitable soil conditioning compost; some of those I listed above aren't readily available in all areas (spent mushroom compost, for instance) so I'd usually recommend composted animal manure. The only time I wouldn't is if you were growing root vegetable crops, because they do not appreciate manured soil.
Therefore, it rather depends what form of compost you've seen that's cheap, and what you're intending to grow in the emended soil, in order to decide whether it will fit the bill for your purposes, or, to quote your question 'whether it's good or not'.