Whether or not there is a "one-way charge" also depends on how the rental company prices one-ways. Discount brands usually keep the same base rate as round trip, with a separate drop charge. Premium brands usually do not quote a drop charge, but the rate for one-ways is higher. How much higher than round trip depends on a variety of factors.

For short distance, long time, the discounters can be cheaper. For everything else, believe it or not, that's often not so.

In either case, if a separate drop charge does not appear on your guaranteed reservation confirmation, one can not sneak in at the counter.

You should look at the total cost, regardless of its components. Do not fall into the trap of focusing on the extra cost for one-ways. For instance, round trip might be $150/week, and one-way might be $200/week. Or roundtrip might be $50/week, and one-way might be $200/week. In either case, the $200 you would pay is what matters, not whether the second scenario appears to be a worse deal than the first.