You have to really shop for a 28-70L. They're very old and it's hard to find one that doesn't have haze or fungus. You will see a few for sale in the $450~550 range, but you have to really look at the descriptions and be very skeptical, a lot of them have just not been cared for and have a serious issue. Haze & fungus being the #1 problems as you shop. A really well maintained one will easily be $500~600, because the market is just not really there anymore, so they're just over-priced. And when you can get a Tamron 24-70 F2.8 VC for $800 it starts to tug at you quite significantly that for a wee bit more, you get a modern lens with image stabilization.
I only say this as someone who shopped for a 28-70 F2.8L for a while, and never committed, because most of them just have too many problems these days, and the going prices in the $500 average range is just not cheap enough to make it a good deal, when for $200 more, you get the MKI or $300 more a Tamron. Regardless of being sharp, it's ancient and if anything is wrong with it, it's basically a total loss.
Shop carefully.
I'd get a Tamron 28-75 F2.8 (~$180~200) and just live with it for a while, save up, and get a Tamron 24-70 F2.8 VC used/refurb when you have the funds, and if you feel the focal range meets your needs.
Very best,