Is a BookBub Featured Deal worth the money, or are the cheaper promo sites good enough?

They're not really substitutes — they're different tools. A BookBub Featured Deal, when you can get one, still has no equal for raw reach, but they're selective and pricey, and for a debut with few reviews you'll likely be rejected anyway. So the honest first-timer answer is: don't start there. Stack the cheaper sites instead. Book your promo across several reputable newsletter sites on the same few days (so the visibility compounds), point them at a discounted or free title, and make sure you have a sequel or a newsletter signup waiting so the burst of readers goes somewhere. That teaches you what a promo actually does for your book at low cost. Keep applying to BookBub over time — your odds go up as you gather reviews and a backlist, and by then you'll know how to make the day count.