As others have suggested, a good flat plate, and a few accoutrements. 48"x36"x4" Grade B granite plates were a few hundred dollars when I got mine maybe 10 years ago. Basic vee blocks, HT cones, a relative height/scribe gauge and a machinist's square with an 8" blade, all at the same time. It was a long time before I got a whipping post.

As to a frame jig, if I had it to do over again (I build in lugs so this is somewhat lug-centric) I'd be using a plywood frame fixture similar to the design found in the Talbot framebuilding book for quite a while (I did so for a few frames), the fixture being used for tube fitup and pinning. Once pinned, thence to the table for an alignment you can count on, and then tacking/rechecking/correcting, prior to brazing. A fork fixture would come long before a frame jig for me; you can use the table but the jig makes it so much easier and quicker.

Expensive frame fixtures are way sexy and production shops need them but for an amateur there are numerous, inexpensive alternative tools and procedures. There is a raft of equipment that will get you farther and faster than a frame fixture and some of it can be home built (brake boss fixture, rear end fixture). A vise is mandatory; putting it on a pedestal, so you can walk around it, is extremely handy though some of the finest builders have it on a workbench corner (as I did for a long time). Drop-out alignment tools, BB/fork thread cutting, HT reaming/facing, BB facing tools, it gets to be a long and expensive list....a fancy, store bought frame fixture is nearly dead last in my book.