Hi kingcrowing,
Horizon with ribbon is $1450+. If you are looking for center with a dome tweeter, I would recommend going with our CMT-340.
An MTM "center" (btw, an MTM is a 2-way speaker) with 5" or 6" woofers using the smaller Sierra-2 ribbon tweeter is certainly possible at around $1500. However, it will not perform well as a horizontal center. Reason being is due to where we must cross the smaller RAAL to the larger woofers. Horizontal off-axis performance will be severely compromised due to wave interference. 4.5" woofers would be the largest I would use (our Duo)
The only true ribbon tweeter that would work well in this MTM configuration would be the RAAL 70-20xram, the ribbon we use in the Horizon and Tower. What makes this ribbon so unique and simply so good is that it is designed to be crossed as low as 1.8kHz. With that low of a crossover point, and when considering the center to center spacing between 6" woofers, horizontal off-axis wave interference will be reasonable.
The RAAL 70-20xram is an extremely expensive tweeter, costing significantly more than the smaller ribbon tweeter. Unfortunately, retail pricing on such a speaker is going to be much higher than your price point. There is no company on the planet offering speakers with RAAL 70-20's for less or even the same as we do, and that is because we purchase these in larger numbers than anyone else and even with that, our profit margins on these models are embarrassingly low.
You can approach Salk, but like what I just mentioned, you can't use the smaller RAAL in an MTM center with woofers any larger than what we already use in our Duo.
Horizon is your best bet by far, both in performance and price.