It’s hard for us to know.
We could estimate the cost of the average free share I don’t have a massive sample but I imagine it’s in the £3-5 range, potential doubled if it’s from a referral.
The next thing we need to know is the retention rate (which you allude to in your post) how many of those users are are sticking around. This is hard to know in the short term and impossible to know in the longer term.
I’d would hazard a guess that Freetrade’s marketing spend is actually fairly modest for the sector.
I think this strategy probably works well for a couple of reasons: it costs almost nothing until people try the app (with advertising you have the first hurdle of getting people to even install it). I imagine retention is higher because people are now invested and the referral system gives them an obvious point of contact to help guide them into the app/investing.
If you want to read more I suggest looking at Customer Acquisition Cost, it can really make or break startups. Without seeing Freetrade’s accounts (and having a crystal ball for future retention) we can only guess.