To answer your direct question: "whichever rate where benefit > costs".
I don't think it's possible to determine a formula a priori, because conversion rate is influenced by many factors, like traffic, design, targeting, funneling, geo location, time of the day, day of the week, month of the year, price (absolute), price (relative to competition), layout (copy, color, images) and many more. As you may notice, there are both quantitative and qualitative dimensions to consider, so I find it hard to believe a formula is even possible.
To better understand what I mean, take a look to this page. It deals with only one of the dimensions (color) and you'll see how changing only this could mean some dramatic increase in CTR (or decrease, of course). I personally love this:
Bing increased their revenue by $80 million dollars by finding the
exact color of blue for their links. Why is that? It’s because people
are used to links being blue. When WWW first came to be, blue was the
color of links. Microsoft engineers working on this also admitted
this: “…it was a shade of blue quite similar to the one used by
All of the above being said, you have to consider your conversion rate as a relative value rather than an absolute value. When you launch your site, you will obviously start at 0, since you won't have any data to compare. Once conversions starts to flow, you'll be able to analyze data and identify issues (if any). With time, you may have something like this (just an example):
Week 1 CTR: 12%
Week 2 CTR: 12%
Week 3 CTR: 8%
Week 4 CTR: 4%
Week 1 CTR: 11%
Week 2 CTR: 14%
Week 3 CTR: 7%
Week 4 CTR: 5%
This will tell you a median you can use to compare further months. You'll see first month's average CTR is 9% and second month is 9.25%. So 3rd month results will tell you if the rate is more or less static or if there's a trend. It will also tell you your users are being more cautious towards the end of the month, so you can plan for that as well.
Bottom line is: CTR is just a metric, at the end of the day, the only thing that matters is the end result. For example, with the same amount of traffic:
- 90% conversion rate on a $5 dollars product/service is way worse than 1% CR for a product/service that costs $100,000.
- 20% conversion rate on a $5 dollars (fixed) product/service is way worse than 15% on $5 dollars (recurring) product/service
- 20% conversion rate on a $5 dollars product/service with $4 expenses is way worse than 20% conversion rate on a $5 dollars product/service with $2 expenses
- If I offer $500 to every visitor that buys my $200 dollars service, I'll probably get close to 100% CR. How is CR important at all if I'm losing 150% for every sale?
and the list goes on and on and on
So again, it's all relative to the many metrics you'll have at hand