To make a Tandem bicycles you have to make the frame strong enough for two. This is complicated by the fact each rider is further away from one of the wheel, which requires the frame to be even stronger. Thus it is hard to produce a tandem for anywhere near the cost of a single bicycle (and that is ignoring the economy of scale you get with regular bikes, which you can NOT get with Tandems).
As to parts other then the frame, Tandems can use either use the same parts as a regular bike or parts with small variation (for example, the hubs can be the same, but you will have to do a modification to permit two riders to pedal at the same time). The wheels will have to be made stronger, for like the frame, they will be carrying twice the weight.
All together, it is very hard to produce a Tandem at the same price as a regular bike with the same level of parts. The rule I use, is a Tandem has to be twice as expertise as similar regular bicycle. Now this price difference goes down as the price of the bike goes up but you are looking at the low end and for $500 I just do NOT see anything that approached the word "Decent".
As to riding with your wife, remember to make sure both are you are riding for the same reason. A lot of couple go to Tandems, but are unhappy for their go for two different reasons. One person (Generally the wife) goes for the Tandem for it is something she can do with her husband. The second person (Generally the Husband) wants to go with a Tandem for Tandems are FASTER then regular bikes with the same level of components. Sometime this dual view works out, sometime it does not. Make sure you and your wife view as to a tandem, while NOT necessary the same are at least compatible (i.e speed while working together).
The Tandems I have seen and liked are all way over $500 (My Favorite was $7500). It is difficult to make Tandems any where near the price of a regular bike with the same components. Schwinn's tandems tend to be sold for twice the price as a regular Schwinn in the 1960s and 1970s. If your family had one, they paid extra for it.
Some Tandems I have seen on the net:
Here is a site with Tandems under $500:
http://www.2wheelbikes.com/tandem-bikes.html?gclid=CKqKsbLk36ECFRE95QodZ2JBLAHere is a website selling a Cannondale Tandem for $995:
http://www.rematchsports.com/Cannondale-Tandem-Bike-56-cmList of Tandem makers:
http://tandem-fahren.de/Mitglieder/Christoph_Timm/builders.htmlMy "Dream" Tandem, just to expensive even for my taste (If you look at the price catalog, remember it is in EUROs NOT Dollars):
http://hasebikes.com/140-1-tandem-pino-custom.html