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.