I'm going to tell you a little of my experience rather than recommend a specific one because I think that it is poignant.
We tried a free assessment with a four agencies in total. The first one said a full assessment was free to British people, but then sent a message back saying sorry it's only free to British people

. The second one was actually recommended by someone (aussiemigrant.com) and they didn't get back to me for ages. Another suggested we go to Oz and try and find a sponser (something my husband wasn't about to do) and then the next one was quick to reply, positive about our chances and had an office here as well as in Oz (A&M Australian Migration). Well at the time that I was getting replies from A&M I suddenly received a reply back from Aussiemigrant after over a month. Aussiemigrant had actually come up with a very good Australian job match for Hubby which would get us a permanent residancy visa, but we decided to go with A&M because of the prompter service and the local office. However, I don't think that A&M would have figured out the job match that Aussie did, because they were looking at lesser job matches which would have meant Hubby taking a step back in experience and likely only getting a temporary visa.
As it happened, we passed on the suggestion of Aussie's to A&M and the help they gave us proved invaluable, because I would not have known where to start in proving Hubby's experience. Our agent double checked everything we did and wouldn't send it until she felt we had everything we needed to get the pass.
Money wise, the total amount we paid was £1800, payable in two installments of £900. However, we were told that if we didn't want to continue with the visa application after the work assessment was done (ie. if we failed it) we could claim £450 of the first payment back. I hindsight I think I might have still claimed it back and continued the visa process alone anyway, as the job assessment was the hardest part for us.
So my personal advice is ask around and compare, don't just rely on the information from one and make sure they are registered with MARA. So far I have found A&M very good, but Aussiemigrant also came recommended and they certainly seem to know their stuff when it comes to trade assessments. There are also some agents on here who seem quite good (Mr Collett from Go Matilda for example). They don't seem pushy and are ready to offer general advice without trying to sign you up.
Hope this helps.
Cara