The bar is open Monday to Friday, 12 noon - 3pm and 5pm - 11pm, and Saturdays 5pm - 11pm only. The cheapest drink on the example menu is an £8 a glass wine (175ml and exclusive of 12.5% service charge, of course), but it would be an experience and bottles are from £32.
There's also good news if you're looking to host a party. The champagne lounge can be hired exclusively from Monday to Friday, but your author has a feeling it will cost you.
Vertigo 42 is in Tower 42, Old Broad Street, London, EC2N 1HQ. For more on Vertigo 42, see http://www.vertigo42.co.uk/