I'm impressed by MapBox's Natural Earth Hypsometric and Bathymetry base layer. The data behind it is public domain and the software stack is open source. There's even some open source support for running the Mapnik tile renderer on Amazon EC2. One might extend it using more detailed elevation data to generate the extra zoom levels that MapBox doesn't host. Pleiades doesn't need to zoom in to street level, but needs zoom levels equivalent to Google Map's level 12 at least. I'm curious whether global tiles at these zoom levels are possible with the service's SQLite-based architecture. The company's $500 per month tile hosting seems pretty reasonable, especially if it includes hosting the highest resolution tiles. Hosting global high resolution tiles at 12-15 zoom levels isn't going to be trivial or cheap, that's for sure.
Mixing of modern and ancient tiles could be a good interim approach. We might use the MapBox hypsometry and bathymetry (for example, or another equivalent tile set) as a base map layer and then overlay custom tiles generated in a matching style wherever we have ancient GIS layers which differ from modern layers. This requires more analysis to identify the tiles at all levels that would be impacted by feature differences – mainly the removal of modern canals and reservoirs, but also some shoreline changes – but in the end requires many fewer tiles tailored exclusively for ancient world map users and less duplicated effort.