I used the Trident sim many moons ago and it worked out at something like £50 per hour......which was unbeatable. I also paid a Monarch F/O (his name escapes me now) who was familiar with running it for an hour or so of his time to take me through the profile I was attempting to learn. Did the trick.
When I got the call for a 'big' aeroplane sim check I went at short notice to GECAT Gatwick (the only availability I could get) and did two hours in a 73 sim before my 'make or break' assessment. It cost me nigh on £800 and I flew it like I had one arm and one leg....! Awful. But, two days later I passed my assessment with flying colours, mainly due to remembering all my errors and working on those in my mind (in an armchair!) before the day. I considered the £800 to be a lot of money............but well spent as it got me from a piston twin onto a 757 course. Personal choice though and only you know how much you want a job and how good your jet handling is. If you want it enough and the opportunity is there on a plate you will make sure you do everything to maximise your chances of success. If you do not want it enough and just approach it as any other selection you will probably not shine and those who do want it enough will. Which one would you employ in the current market?
I know 2 people who have failed Britannia sim assessments recently and who are kicking themselves for not getting some time in before the day. What you have to understand is it could mean a few more years striving to get yourself even another shot at a sim assessment for another large airline. Put it in perspective and decide how hard you are going to push to secure the job. Don't let an opportunity pass for a reason that you could have controlled. If you don't get selected let it be because others were better, not because you screwed up.
Good luck.
PP