Your "best value" in a conventional 28" or 700c tandem would probably be a used or even a new Cannondale. The Medium/Small Cannondale MT and RT models have a standover height (inseam) of ~28.2" and top tube lengths of 22" (MT) and 21.5" (RT).
The smallest frame size in a conventional 26" or 700c tandem would be a small Co-Motion Mocha. The Co-Motion Mocha (26" enduro) in the Small size has a standover height of 27.4" and a top tube length of 21.25". Their road tandem Small size has a standover of 29.8" and a top tube of 21.75" which is a a pretty big jump up in size. However, Co-Motion offers custom sizing for about $300 which if you are a non-standard size team can be money well spent. Moreover, the upcharge for custom sizing can sometimes be negotiated.
Burley's small Mountain and extra small Road tandems have a standover height of 29" with 21.5" and 22" top tubes. I believe, but am not certain, that Santana's small size frames are similarly sized. Santana has some funky fees for custom sizing (admin + sizing) but have been known to offer up either custom sizing or custom paint jobs in the past.
Finally, there is a full-on custom. While many buyers automatically assume custom = expensive, the truth often times is quite different. In this regard, if you're used to premium quality bicycles a Bushnell custom would be worth looking into. Every Bushnell is a custom and the pricing is comparable to stock production tandems. Tandems East (www.tandemseast.com
) in New Jersey and Tandems Limited (www.tandemsltd.com
) in Alabama are both Bushnell dealers and if there is not anyone in your local area who is a dealer for Bushnell you can most certainly contact Dennis Bushnell directly (BushCycles@aol.com).
So, I think that's your playing field for conventional tandems. As others have already noted, Bike Friday's 20" wheeled "family" and travel tandems and Co-Motion's new Periscope offer a wide range of size adjustability and would be worth consideration if you don't mind a somewhat "different" looking frame style.