A type rating B737 can easily be done for well under £15,000 on condition you do the training as a crew iso on an individual basis!
At most airlines they team new F/O-trainees up with their Captain-trainees, resulting is a much more efficient and quicker training.
As far as I know the Ryanair system, they simply can not do this, since only their F/O's need the type rating course (the captains joining Ryanair all bically have their typerating from a previous company).
This unnatural flying environment results in an unnecessary slower, longer and thus much more expensive training for the F/O trainees.
How much longer depends of course on the individual capacities of the trainee, but as a guideline I can tell you this:
Occasionally it happens that airlines like LH, AF, SN, etc. put 2 F/O's together for their initial training, but then it is standard practice to include at least 3 additional sim sessions. If you know a normal training can be completed in around 10 sessions, then these 3 extra represent 30% extra time,thus also cost!
If Ryanair charges you 15,000£ it's a real rip off; it can be done for as low 10,000£!