You must mean due claimant by an unsuccesful respondant (maybe I'm resisting this concept a little here) ?
They are all free to set their own fee structures. WIPO charges $1500 for a single member panel complaint. If the complaint is withdrawn before a panel is appointed, such as by the parties agreeing to a rule 17 transfer, then the complainant is refunded $1000. If the Respondent pays $2000 for a three member panel proceeding, but the parties agree on withdrawal, then the Respondent is refunded the $2000 (and the Complainant is refunded $1000).
Do you happen to know where I can find out more about refund policies of individual UDRP providers?
All of their fee policies are explained at their websites.
So the way I understand it, when the domain is cancelled or transferred this amounts to no respondant costs incurred.
The UDRP providers don't have any mechanism for making the Respondent pay anything. So if a Panel for which the Complainant paid (whether they paid for a single member panel or a three member panel) decides to deny the complaint, cancel the name, or transfer the name, it's not as if the UDRP provider has any way to collect a fee from the Respondent. Most Respondents don't show up anyway.
Unleses the UDRP provider has a fee for implementing Rule 17?
None that I know of. It would be kind of silly to charge a fee for that. Because the Respondent could then simply file a Response saying, "Please transfer the domain name" and most panelists will simply do that. Now, the NAF will pocket ALL of the complainant's filing fee when that happens, but...
I really don't understand your interest in the topic. If you are a domain investor, it's not as if you get to pick the UDRP provider through which someone files a complaint against your domain name.
Is there a single source you might be able to point to?
Yes. Me. I charge a reasonable hourly rate or can quote a flat fee for answering your highly specific questions about UDRP provider policies, but I don't offer refunds.
Either you have received a UDRP, in which case the only policies of interest are those of the UDRP provider in qutestion; or you are contemplating filing a UDRP, in which case the edge cases in which fees are refundable is hardly your biggest practical concern.
But I don't understand the general practical value of this discussion.