Yn ôl asesiadau a gynhaliwyd gan gynghorwyr morol ac eiddo, a chan swyddogion, bydd y gwaith galluogi yn costio £136 miliwn yn ychwanegol. Aed ati i adolygu’r Achos Busnes ar gyfer Prosiect yr M4 er mwyn cymryd y costau hyn, yn ogystal â'r ffaith bod bwriad i ddileu'r tollau ar Bont Hafren, i ystyriaeth. Mae’r Achos Busnes hwnnw’n dal i ddangos bod y prosiect yn rhoi gwerth am arian. Aseswyd, yn ôl amcangyfrif ceidwadol, fod y Gymhareb Budd a Chostau yn amrywio o 1.7 ar gyfer buddion uniongyrchol i drafnidiaeth, i 2.3 (gwerth da am arian yn ôl canllawiau'r Trysorlys) pan gaiff buddion economaidd ehangach eu cynnwys.
The enabling works have been assessed by maritime and property advisors and officials to cost an additional £136m. The M4 Project Business Case has been reviewed to take these costs into account, as well as removal of the Severn crossing tolls, and continues to demonstrate that the project represents value for money. The benefit to cost ratio has been conservatively assessed as 1.7 for direct transport benefits alone, rising to 2.3, considered high value for money, when some wider economic benefits are included.