There's several candidate mods you could go for.
The Vamo, the SVD, the SID are all decent VV/VW mods at under £50. However with all of those you will also need batteries and a charger. that will add £40 onto the price, but once you have those you don't need to buy them again and they'll prove useful later too if/when you upgrade to other mods.
If you want a good quality mod for under £50 all-in there is basically a choice between which colour of MVP you'd like.