Actobotics sells some fairly cheap base kits:
The scout is $170. It includes motors, but does not include electronics. I believe the motors do not have encoders, but Servocity does sell a version of that same motor with an encoder for about $20 more per motor. I'm sure you could call them and request upgraded motors.
This base has bolt holes that are compatible with the Actobotics construction system so it would make it very easy to build a superstructure to support the OPs equipment.
Depending on the scope of the project they sell a bunch of interesting simple chassis or some fairly complex chassis with more space for components.
One of our FTC teams switched to the Actobotics building system a couple years ago and have found it to be a really well designed system. They have a bunch of specialty COTS parts to be able to create really unique mechanisms. They have mounting plates that are compatible with standard servos, motors, etc that make it really easy to assemble. The parts are not as cheap as Tetrix, but they fit together a lot more precisely and solidly.
Our club has also built a bunch of very simple holonomic drive robots using some of our older FTC electronics that we don't use any more, 4 motors mounted diagonally on the 4 corners of a square board with direct drive Tetrix omni-wheels. We use these for outreach based on a game we saw at worlds in 2016 that another FTC team, the Lazybots, had developed for outreach. We copied the Lazybotts plans for their robots which are posted on their website:
Click on the 'more info' link at the bottom of the page to go to their google drive where they have the plans and construction instructions for their robots. Obviously, you could make these as large as you wanted by just making the square plate larger. These robots are way under $200 and provide a very versatile platform if you have a flat, indoor space that you are using for your project.