What statuses do you currently have? Can see if you have anything that will eventually get you to MLIFE status since Gold provides a 25% discount off rooms and MLIFE Plat provides a 30% discount off rooms.

May is a semi-busy month, depending when you go. There are several big conventions going on each week in May. When visiting Vegas, try to look at convention dates first and avoid going during those days/weeks. Definitely avoid Memorial Day weekend as hotel prices are highly inflated.

For Vegas weekends in May, AND wanting to be on the strip for under $100, is going to be pretty tough, but not impossible. Might have better chance if it's only weekdays. Just a quick glance using CET flex calendar rates, you can stay at Flamingo, Linq, and Rio(off-strip) during the week of May 12th-16th for all nights under $100 (avg around $60-$80 a night). Once you get to the weekend, all the weekend nights, even at the lower end properties are around $200 a night. If budget is a big concern, I recommend downtown Vegas/Fremont area. You should be able to stay for under $100 a night at most of the properties there. Only a 15min cab ride away.

If you elaborate more on what you and your SO like or not like to do, I can recommend some activities.