It's true you can save money by paying for just two months on the monthly payment option, but realistically, two months isn't like to do that much for you. Using reader magnets for signups works, but you'll likely only get a few each month, at least in the beginning.
Is your email list going to be worth spending $100 per year on? Maybe not at first. Results differ a lot. But it is definitely constraining not to have one.
Keep in mind that you get more than newsletter signup promos. You also get sales promos (which usually work best for discounted products). When I do one of those, I find the conversion rates from clicks to buys are normally pretty high--anything from 15% to over 50% (based on taking the difference from the previous month's sales over sales during the promo). (I wish I could get that kind of conversion on Amazon ads!) Of course, that does depend on how appealing the book(s) you are promoting is (are) and whether or not it's a good fit for the particular promo. Good covers and descriptions are essential (as they pretty much always are). Undiscounted books will still sell, but then the conversion is more like 3-5%.
It's easier to get a positive ROI from Bookfunnel if you use it for sales as well as signups. For the fee, you can participate in as many of those as you want (though realistically, you probably shouldn't do more than one or two a month to avoid exhausting your new mailing list subscribers and social media fans. Promotion sponsors will be understanding if new authors with no mailing list don't send many clicks to the promo, but you need to be able to generate some as time goes on, and bombing fans with too many promos at a time reduces the response rate considerably.
Another thing to consider is that Bookfunnel has good tech support for people who download your freebies, for example, helping if they have a problem loading them on their devices. This becomes more helpful if you decide to use BF to deliver ARCs or if you want to use ebooks as giveaway prizes.
As I think about it, maybe your best play is to sign up monthly for a couple of months and see how it goes. If it produces good results, you can think about a longer commitment.