See the link in the wiki. Prices vary by miles, but someones has to have at least 85K to be eligible to boost to EXP. (And it doesn't matter if you are currently EXP or not; if you have at least 85K miles or points (or 85 segments), you can boost, so it's not a renewal program per se).

85,000 - 89,999 is $2499 to boost to EXP.
90,000 - 94,999 is $1799 to boost to EXP.
95,000 - 99,999 is $1199 to boost to EXP.

I agree it's a lot of money and most people would be best served to do a mileage run instead.

Similar boost programs (at lower prices) are available for those near Gold or Plat to allow them to boost. There's also an option (for current Golds and current Plats) that are not with 15K/15 segments to renews their current status for a price. That renewal option does not exist for the EXP level. So if someone is currently EXP but has only 84,000 EQM/EQP this year, for example, this elite 'buy-up / buy back' program will not help them. There is, however, an option for current EXPs who are more than 15K short of Plat (so have less than 35K EQM) to pay to 'renew' Plat level status.