Had our leech field replaced last summer. The septic tank itself was fine.
Initial testing to see if it had failed was $110, applied toward the total cost of replacement if needed.
To add a leech field was $5750, minus the $110, so $5640. This included one pumping so they could attach new lines. Our ground was so saturated that it had to be pumped twice more. I think that was an additional $400.
They installed a diverter valve and we essentially have two leech fields now. I can wait about six years and we can switch to the old, just have to make sure the biomass has decomposed and the old system working before doing that.
They also said if we needed a new septic tank, it's an additional $1800.
This price included a rough grade of the soil afterwards. My husband fixed our sprinkler system for around $200 and it cost an additional $1400 to have the backyard finish graded, additional topsoil and seed.
I would take lots of pictures, before, during and after. Especially of the sprinkler head placement and of the utilities flags. This would have made the placement of the sprinklers easier. Our utility company came out and placed flags for all of our lines--but neglected to place flags for the electric line of my neighbor's that cut through our yard. They tried to place blame on the septic people, they would have had to pay for the new line. Luckily I had pictures that proved otherwise.