i've done two heartsy deals, both of them $18 for $40.
the first time, months ago, i sold 95 vouchers.
the second time, last month, i sold 113 vouchers.
the second time was an additional $10 to VIP's and those VIP sales made up close to 75% of my voucher sales.
after having been through it, if i ever decided to do another deal, it would be $22 for $50 for VIP only.
your price point is higher than mine, though. maybe you could do something like $28 for $60 and that would encourage people to spend over the voucher amount a bit?