Define reasonable.
Is this your first tandem for you and your riding partner? If so, I'd heartily recommend a used tandem for your first bike just to make sure that tandem riding is an activity you both enjoy. If you don't both enjoy tandem cycling, then disposing of the bike won't be so financially painful.
The Dolan frame from the UK starts at about $800 US. From what I could find online, it appears that Dolan sources their frames in Taiwan or elsewhere in Asia. That being the case, if you want a new frame from a dealer who cannot support you in the USA, you can look to importing from a Asian manufacturer. Performer bicycles of Taiwan is primarily known for recumbent bicycles here in the USA and there have been companies importing them in the past. They also do direct sales so you can self-import. They do make several models of upright tandems at reasonable prices. Their website lists complete bikes but perhaps they will sell you a tandem frame alone.
https://www.performercycles.com
Our first tandem was a Nashbar rig purchased new in the late 90s for about $1k. At the time it was the most I'd ever spent on any bicycle. Unfortunately, that relationship didn't last (not because of the tandem) but I kept the bike and found a new riding partner. We enjoyed cycling on single bikes and the Nashbar tandem but we discovered the Hase Pino tandem and haven't looked back. We log very few single bike miles anymore. The Pino is expensive but we've really enjoyed it over the last 6 years and I'd do it all over again. I'm not suggesting that the Pino is the right bike for you, rather I'm trying to make the point that finding and buying a tandem that really works for you and your riding partner can be cost effective even if the amount spent seems high as long as the expenditure doesn't keep you from paying for important things like housing and feeding your family,
Good luck!
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