The vent system is not inexpensive I've heard. Depending on how far they need to run it, that can be a substantial cost. It is often recommended to install a tempering valve on the outlet...those aren't particularly cheap, either in either materials or installation complexity. Depending on the design, making provisions for deliming the thing adds complexity and therefore costs for both materials and labor. Depending on your local codes, you could be looking at at least several hundred dollars for permits.
Keep in mind that you may need to upgrade your gas service - a high volume tankless system can draw more gas than a large furnace. So, depends on what needs to be done...that price may be cheap. Some of the tank type WH can approach the efficiency of the tankless, so energy cost differences might not be that big.