Should I use monthly subscription or one-time payment? How much should I charge?
Can you share some general advice on what is the best way to decide or what to avoid?
The general advice is: testing. Drive traffic to your landing pages and see which one converts the best. With that I mean: which source of traffic coverts best AND which landing page design works best. HINT: hire a professional copywriter, he will work wonders for you there.
In any case, test and thus find the best performing combo.
You should put some effort into the presentation of your company. Why should anyone trust you with their data? This is a very big question, that you MUST address. (Minor nitpick: I've come across websites of small firms and they didn't even bother to tell in which city they reside. This is of course a form of mindlessness, which told me everything I needed to know to not do business with them. Details matter.)
I suggest to make a monthly plan, with an option to pay annually with a 1 or 2 month discount.
How much are other similar projects charging? Don't put a very low price.
Read some advice from patio11: https://news.ycombinator.com/item?id=13843743 (The whole discussion may be useful https://news.ycombinator.com/item?id=13840282 )