You've probably seen it but Walkhighlands has a stage by stage guide to the
Scottish National Trail with OS mapping which will give you an idea of timings etc. Costs will depend a lot on what you plan for accommodation, wild camping is possible for much of the later stages (and also some bothies - they are all mentioned in the route description) and some of the first ones, maybe budget £7.50 - £12 for campsite, £17-£20 for hostel and from £35 for B&B, pub meals around £10-15, takeaway fish and chips around £6 - have a great trip.