I live in Australia, I think concreting costs are similar.
With that range of quotes, I suspect they are offering different things
- so I would ask some questions about what they are offering to do.
The cheapest quotes look very cheap (about $2.50 a square foot) and I
suspect he hasn't allowed for a compacted crushed rock bed and steel
reinforcement in the concrete. $2.50 a square foot seems like a
reasonable price to turn up on site, scrape the ground a bit, build a
timber frame and call a concrete truck to pour a slab.
For $5 a square foot you'd expect to get a bit of site preperation
(like some levelling and a bit of grass removed, a crushed rock bed and
steel reinforcement in the concrete.
What sort of job you want depends on how long you want the pavement to
In my house there's a lot of concrete paving done about 35 years ago
with no steel reinforcement and clay soil. It's cracked all over the
place with soil movement.