The Costs of Publishing Monographs: Toward a Transparent Methodology was published earlier this year. It looks at the actual costs to publish rather than charges made by the publisher, but given the very limited size of the OA monograph market at the moment, this may actually be more useful. Given the relative rarity of OA monographs from mainstream publishers, it's likely that many are specifically negotiated on a case-by-case basis and general numbers are not available.
In terms of published charges, Ubiquity offer some indicative figures here, with the caveat that they will vary in practice based on length and other issues. These are probably at the low end of the market, and a traditional publisher is likely to charge more. There are more indicative figures in this report from OAPEN, ranging from under £3000 (Ubiquity) to above £10,000 (Palgrave).
As noted in the comments above, this will vary a lot by discipline and by the type of book - a scientific textbook and a humanities monograph are very different things with different markets, and this will influence what publishers are willing to arrange.