I hate to be the "it depends" lawyer here, but it really does depend on a couple of things.
I have successfully defended a UDRP for $500 at the minimum, and I have successfully defended a UDRP for $7,500 at the maximum.
Some cases are more complex than others. Some cases are almost exactly like cases I have done before, and the brief is largely a cut and paste job. Some cases involve a lot of evidence, and some don't. Sometimes the client is more or less cooperative and flexible. I work odd hours. If I can ask you a question at 1 AM, and not have to interrupt the flow of the work, that's more efficient than having to schedule telephone calls.
If you have a rare instance in which your position is outstandingly meritorious, then we might consider whether a three-member panel is necessary. However, in nearly all cases I recommend using a three member panel which introduces a base cost of $1350 at NAF and $2000 at WIPO.
After filing the response, the other side may introduce supplemental materials, which may or may not merit a detailed rebuttal.
Once in a while, I have had engagements in which I review the complaint, write an outline of a response and provide relevant citations, and the respondent has used those material to write their own response, which I then review. I only do that rarely because sometimes that takes more time than writing it myself from a clean sheet.
If the name is not worth either your time, effort, or expense to respond, then you still will want to consider following the unadvertised procedures provided by NAF and WIPO for early transfer and termination of the case. Even if your position is meritorious, but just not worth the hassle, it can be worth avoiding a decision against you, so that it will not be cited when someone decides to take on one of your more valuable names. Putting together the appropriate forms and correspondence with the complainant, the provider, and the registrar, is something that my assistant Cristin does with high administrative efficiency. (cristin -at- johnberryhill.com)
(also, directing first time correspondence to cristin is generally more efficient than sending things to my heavily spammed inbox)
Like the sign says - Dumb Looks Are Still Free!