Reddit Ads for a small B2B SaaS: subreddit targeting or interest categories?

Picture a B2B SaaS with about 1,500 a month to test on Reddit. Targeting is mostly subreddits or interest categories, and that choice matters more than the creative. Interest categories give you volume, but the people in them are often students and hobbyists who will never buy a seat. Subreddit targeting feels more on-topic, but on smaller subs frequency climbs fast and the list can be exhausted within a week or two. My hypothesis is to start with a short list of mid-size subreddits where people actually talk about the problem the product solves, and judge each one on cost per signup over about two weeks, not on CTR. Does interest targeting ever hold up for B2B on Reddit, or does it just burn budget?

Agreed that subreddits beat interest categories for B2B, but two weeks of cost per signup on a $1,500 budget is going to be noisy. Say your target is $60 per signup. A subreddit that gets $300 of spend over two weeks gives you five signups, and one good or bad week swings that number a lot. A rule I'd use: don't compare a subreddit until it has budget for about 20 signups at your target CPA, so roughly $1,200 per cell at $60. With $1,500 total, that means running one or two subs properly, not five. You'll learn less per sub, but what you learn will hold up. If you can, judge on trial starts or demo requests instead of plain signups, since free-tier signups from Reddit tend to be the students and hobbyists you mentioned.

Pair Iris's 20-signup floor with a frequency cap. Once a subreddit's frequency passes about 3 in a week, stop it and read the result. Past that point you're mostly paying to reach the same people again, so the CPA stops telling you anything about the sub itself.

Frequency cap makes sense, but a fixed 3 doesn't. The right number depends on audience size. On a sub with a few hundred thousand active readers, 3 a week barely touches the same people twice. On a tight niche sub, you can hit that by day three. So set the cap based on reach instead of a rule of thumb.