To some extent, this is going to depend on the lids required by the manhole material, and local codes. In our area, for example, both concrete manhole covers, as well as bolted down plastic covers are acceptable. That being said, either way it shouldn't be terribly expensive, if it is only the lid that needs to be replaced. For example- I just had a pump tank and elevated drain field added to my system. The tanks that were put in had plastic corrugated manholes, and I opted for plastic lids (the manhole can then be brought down to ground level, and you can mow right over the lid). IIRC the lids themselves were $75/each or so. They just use a 6" lag screw that goes into the plastic manhole (if it is a matter of simply replacing existing plastic lids, you can probably do it yourself).
From my conversation with the installer, he said that using the concrete lids (same type that we have for the primary tank) would be around the same price. I have no idea what the old steel lids cost, but I would guess they are a bit more pricey.
In terms of the labor- if it is just replacing the lids, and no work needs to be done to the manhole, I would guess an hour job at most. If the existing lids are plastic- just unscrew the lag screw, pull the old lid off, put the new one on, and screw down the lag screw. If it is a concrete lid, just dig up the lid (ours were a couple inches under ground, except for the PVC pump tube), pull up the handles, pull off the old lid, place in the new lid. Throw the dirt/sod back on top.
This sounds like your leach field is possibly blocked. That's the disaster scenario where you not only need the tank pumped, but the yard dug up and a new field placed.
Water from your laundry coming up in your tub is a sign that it's not draining to the septic system. This is either because there is a block in the system before the tank or because the tank is unable to accept any more water as it is plugged on the discharge / leach field side.
When I opened up the clean out pipe clear water came out (about ten gallons). I shoved the hose down about 25 feet and it was running full force but it seemed like all the water coming out of the hose was draining back out of the clean out pipe.
... and that's the sign that it's doing option #2: the tank is full and failing to drain.
Best Answer
The pump and pipe (force main, being on the far side of the pump) size should be specified by your septic system engineering documents. While I feel similar to the commentators about pumping sewage, sometimes it's the only way (house on a lake, for instance.)
I would strongly suggest both the "two pump and alarm" method (preferably with a pump controller that uses both pumps in an alternating fashion - having a "spare" sit there while the main pump works tends to lead to having two pumps that don't work when the main one breaks) and a mounting method that permits the pump to be pulled out for service (typically the pump is on rails, and the system automatically valves off the pipe when the pump is pulled out.) They are disgusting and expensive enough to repair without setting them up in a manner that requires a pump out and someone going into the hole to fix them.