A packaged treatment system generally costs more to buy and to run than a simple septic tank, because it includes secondary treatment equipment, uses electricity and needs an annual service contract. The overall installed cost still depends heavily on the percolation area and site works required.
When comparing quotations, look at whole-life cost rather than purchase price. A treatment system has ongoing costs — electricity to run the aeration or pump, an annual service visit, replacement parts and periodic desludging — that a septic tank does not, but it can be the only viable option on a constrained site.
Quotations should be based on the site characterisation report, because the percolation area design flows directly from the measured percolation rate and the depth of subsoil available.