There is no cost to you. Some companies do give the candidate a small amount to cover travel costs but this is entirely optional.
CITB staff will complete the majority of the paperwork, and on completion of the evaluation at the end of the placement, eligible employers will receive the £200 support fee automatically.
You should inform your insurance company that you are offering work experience to a young person. Although unpaid, they will be covered by your Employer's Liability Insurance like any member of staff.
A separate risk assessment is not required, as long as your existing assessment already considers the specific factors for young people. Furthermore, there is no requirement to re-assess the risks each time you take on a new work experience student, provided the new student has no particular needs.