in the perfect world, one should not have to spend money on sim prep for a job, I agree.
A couple of things to think about. The 767 is first generation glass, the sim is very much based on that, some have no speed tape, just the old fashioned round ASI. The scan pattern required is different to a modern glass cockpit and this catches a few guys out. Sim prep may help to avoid that one. Plus getting the procedures and calls perfect BEFORE arriving is not a bad idea.
How serious are you about this job? If it's a shot in the dark, then saving money may be an option. If you are very keen and serious on getting this job, you'd be doing yourself no favours by skimping on the prep.
The agency may sound like they are trying to fleece you but I reckon they are just trying to get as many guys through, thus increasing their income.