It is hard to always keep focused when you're hour building, but you can practice a lot of the manoeuvres you'll need to do on your CPL - especially visual navigation. None of the CPL syllabus is anything you haven't done before, you just need to make sure you're very current at it. My CPL school had a different technique for teaching Nav than I was used to, so a few trips getting to grips with it before the course started would have saved me quite a bit of money. If you do your hour building on the same a/c you'll test on it will help too.
On the cost front, I didn't spend much beyond the minimums, although flew an extra sortie on the IR to avoid having a three day window between last trip and test. Also, I went over hours on the CPL (the CAA reduce the minimums to 15 if you have an IR but I was nowhere near ready after 15 hours) which cost more. I spent a lot more on food and accommodation than I expected and spent 8 weeks in a B&B during my multi and IR so the costs add up.
I support HWD's post (especially the comment about age) - £19k with a buffer of around 30% for unexpected problems is a good guide. I spent around £21,000 on the Multi/IR/CPL but went to a more expensive school. I probably spent an additional £180/week on living expenses - sounds a lot, but it didn't get me much. I would need to add on a return ticket to the US too and an MCC. Remember
Doing it all at a school near your home will be a big cost saving.
Hufty