Peter makes a good point about talking through your above challenge with your supplier. Do you have a thought on what you can afford and asked them scope out what they could do based on this? I would also be asking/checking what measures you have in place for this activity and how you tie it directly to sales. If you tied their performance to sales generated would that be a better model for both of you. If you win they win? It could be looking at what % of your marketing budget was to turnover and use this ratio as a starting point.
Secondly what offers can you give you to get people to invest that there is benefit to them? There is a local clothing store I often shop at and during lockdown they came out with an offer of purchase $250/500 and get a $500/1000 voucher to spend. I bet they are also using this opportunity to up-sell when their clients are in store.
Finally, in terms of taking it over for yourself, there are a few things to ensure you can articulate well (1) who is your target market? – normally 80% of revenue will come from some core customers (2) which platform are they likely to be on? This will be critical if you need to boost posts on Facebook (3) what’s the key problem you are solving for them? As this will influence what you say and offer to them. I’m assuming the marketing agency you use will be able to answer these questions and if not I would definitely be challenging them on the work.
Good luck with your conversations.