Well, the counterweight from Michael Lim and the Tangospinner kit for the mdf platter would run $225 + shipping.
Its more than I wanted to spend but Michael said biggest improvements are with counterweight and pulley / belts.
One thing I do like with the Rega tables is the ability to upgrade over time vs. Having to have $1,000+ in one shot to dump on a high end table.
I agree that the counter weight, belts and pulley can make big changes in the sound. I would add the subplatter as well. For me, it was a huge improvement in solidity of bass, clarity and timing.
First, what I wanted to do was get my TT spinning as accurately as I could (without a motor change). To do that I felt the bearing change, subplatter and metal pulley were all important to get changed to something that would have, hopefully, better tolerances than the original.
Next, I wanted to help the balance of the arm and to do that is a under hung counter weight.
I've done the first two and now I want to get the bracing next then the adjustable feet. I don't know that I want to replace the plinth, but I have considered it. The platter, I believe would be a worthwhile upgrade, but I want the delrin and not the acrylic. My last TT had a delrin platter and I think it did a lot of things right.
That's why I love the fact that you can upgrade different parts of your table over time. Like you, a $1,000+ is not easy to get together in one shot.