Once on oDesk before the merger and twice on a similar site I was asked to write test articles for a nominal fee. In all cases I was paid the $2 or $3 for the 1000 word test articles on a specific given topic and that was basically that. Never hired or had continued work.
I have run into like requests here and I assure you I am not trying to tell you how to run this site. As food for thought I will say that I have become suspicious of these type of jobs.
I truly believe this has become a vehicle to get quality work cheaply and I do not feel it is fair to the multitude of talented writers here.
Again, just food for thought.
You are free to tell them you will be happy to test at your usual,or,if you like,adiscounted rate. Nobody forces you to do testing at a fraction of your normal rate. I have done 2 paid tests the last month, I discounted my rate some for both as they really were "tests" and not in any way productive (as in nothing usable is or was being produced.) One even insisted on me running the tracker for the final 1 hour interview,,,,,
They are scammers, they give several applicants different subjects for the 'trial' and get all their work done very cheaply.
Often that have loads of jobs advertised with no, or very few hires and a tendency to interview loads of people.
I've fallen for it in the past, as have many others. Just put it down to a learning experience. Just be wary of these type of jobs in future.
Asking for a trial article is reasonable and I still do trial jobs but only at a normal rate.
Then there are the posts that ask for three story ideas with your proposal. Same thing. Ideas in a writer's world are currency. These should also be flagged as asking for free work.