I use bonus weeks all the time with SFX. As previously mentioned, the pricing depends on the unit size (Studio/1BR/2BR/3BR) and how far in advance the check-in date is. Typically things that are 6+ months away are $899/week for a 2 BR. there's a $200 upcharge I believe for Grand Luxxe. there's also the $75 resort fee (Vidanta charge when you check out). Depending on one's MF/deposit, it can be higiher/lower to use a bonus week vs. an exchange week. For me, it's about a break even.
If I deposit my 2400 point Hilton week, pay the $59 booking fee and the $64 or whatever Guest Certificate, plus my MF/point, and use it to exchange into a 2 BR Grand Bliss, it will cost me (plus the $149 SFX exchange fee): $680
Obviously if I have to deposit my 3400 point week it's a bit more at around $980.
If I use a bonus week, and it's more than 6 months out it's $799 (I'm a diamond member so I get the $100 discount off of bonus week and reduced exchange fee)
But if I can get the bonus week in under 90 days (again, for summer you're probably OK, but I wouldn't count on this during high season) it's $699, which is almost the same as my exchange week.
Bonus weeks expire in 1 year. Exchange/deposited weeks expire depending upon membership level and future deposits.