You are using a Canon 6D which is a full frame body.
The cheapest L lens is Canon EF17-40mm f/4L USM,
pricing wise do check with various as Canon site
states $1.2k.
As it is a full frame body, you won't be able to use
APS-C lens which are much cheaper at around $1k.
In the first place, if you are budget conscious should
have gone for a APS-C body instead.
At least APS-C lens are more affordable than full frame.
Even if you were to opt for other brands full frame lens
the pricing can be quite steep.