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I've done this before and you can put a range of effort into it, from nearly zero to a lot. With little dev effort and zero BD effort you can place ads from ad exchange aggregators and they source ads automatically. That won't be the best revenue per impression but it's easy to get going and low-to-zero maintenance. On the higher side you can of course have a dedicated BD team sourcing 1st party ads, which are likely more native and higher RPM, but that of course requires much more effort.