Costs
We don't publish septic tank prices as a list, because two jobs rarely cost the same. What we can do is show you exactly what moves the figure, so any quote you get makes sense.
Your quote should break down every one of these, so you can see where the money goes.
Comparing quotes
The cheapest quote often isn't cheaper once you add what it leaves out.
Check that every quote names the system and its size, includes the drainage field or outlet, covers removal of the old tank, says how the garden will be put back, and includes Building Regulations and any permit. If one doesn't, ask why.
Questions
Because the biggest cost drivers, like ground conditions, access and the distance to the drainage field, can only be judged on site. A phone figure would be a guess.
The unit itself usually costs more, and it needs electricity and servicing. But it can need a smaller drainage field, or none where a watercourse discharge is allowed, which can balance the total.
Check each quote covers the same things: the system and its size, the drainage field, removal of the old tank, reinstatement, approvals and commissioning. A cheaper quote often leaves one of these out.
Tell us what's happening and roughly where you are. No need to know what type of system you have.