If you're willing to book in advance and can select a specific train, then there are fares as low as 3-5GBP from LGW to and from Victoria, on the Southern services.

www.nationalexpresseastcoast.com will allow you to book and pay for tickets in advance, with no service fees charged ^ with tickets available for collection at the station. Type 'Vic' in the search box at the NXEC website and Victoria will obligingly pop up, as it's not one of the usual stations served by that train operating company.

But as far as I can tell, you must ride on the train at time X rather than on any Southern service; allow a bit of leeway for this when booking the train from LGW to Victoria.