I always shy away from stuff like this because I view it as one of two things. Either I'm getting ripped off if I pay for Plus, because 8x the cost to me means your margin is huge, or I'm getting subsidized by you with the Pro version which means I can't rely on it lasting long term.
I also dislike daily limits for search. My search usage isn't uniform day-to-day. I might go most of the month without searching for anything and then do a ton of searching over 2-3 days when I'm trying to learn something. So I'll be idle most of the month and then not have enough searches on the days I actually want to use it.
I prefer the model used by a lot of pre-paid services. Let me deposit a chunk of money (ex: $20-50 minimum) and charge me per search until my money is gone. That way I'm not "losing out" if I don't use it every day and I can "burst" as high as I want when I'm trying to learn something.
If the pricing is based on a certain amount of loss (on my side) from the use-it-or-lose it model, I don't like that. I want simple, fair pricing, not a complex pricing scheme where the primary purpose is to get me to overpay for my usage.