Robo-advisers are spending up to £500 on acquiring each customer, according to the latest research.
A report by consumer financial education website Boring Money suggested a huge range in the amounts brands are spending on customer acquisition.
Boring Money interviewed more than 20 providers to produce an overview of how much is spent attracting new customers and coverting them to the company's investment service.
It found were banks typically spend less than £100 to convert existing customers into investors who use the banks's advice services.
However, robo-advisers typically spend between £200 to £500.
Investment platforms spend less on average, it found, although depending on how many parts of the business the company is trying to attract a potential investor to it can cost up to £350 per person.
Holly Mackay, chief executive officer at Boring Money, said: "We see a huge disparity in spend.
"At one end of the spectrum, the banks have relatively low costs when it comes to converting existing customers to their investment propositions.
"Newer robo-advisers have to spend more and in some cases the spend goes above £400. This is clearly unsustainable with revenues of less than 1 per cent and average account sizes of £20,000 across the board.
"Some life companies are spending crazy numbers, justified by consolidation potential which as yet has not materialised and account sizes remain low.’’
Aj Somal, chartered financial planner at Birmingham-based Aurora Financial Planning, agreed the acquisition costs were too high.
“With regards to robo-adviser spend on new client acquisitions, a cost of £400 plus is clearly unstainable based on case size and fees.
"There will be robo-advisers that fail to get critical mass to make a profit, and there will be inevitable consolidation in the robo-advice sector over time.
“Robo-advisers have to spend more, as they do not have a brand name, and hence need to build trust with the end consumer, whereas compared to the banks, they have long standing relationships with clients.”
The data was published by Boring Money, which is an independent business set up to help people make smart investment decisions.
It found that the battle for the non-advised customer is gearing up as robo-advisers launch and the banks give online investing greater prominence.
Barclays’, for example, has recently added online investing to its main online banking landing page.
The report also analyses which brands customers would most like to invest with.
Despite having no investment advice propositions in the market place today, retailers polled well with consumers as potential providers, as did the banks and building societies.
Ms Mackay added: "The current investment platforms have spent the vast majority of budgets on digital and conversions, and very little on brand.
"It's going to be hard for them to compete in the tougher battle to convert savers into investors.’’
Robo-advice is on the rise. LV said it plans more innovation as it looks to grow its business following the success of its robo-advice and automated underwriting systems.