What is the most economical way to drive an array of single coil latching relays?
Someone asked a question about a mystery circuit here a while ago, that lead to a pretty damn clever way to single-coil latching relay with one IO line.



the 100Ω resistor is the relay-coil. Note that you will need to size the capacitor (220 uF in the image) to properly drive your relay coil, and you should test the circuit with a fairly worn-out relay, in case the required coil current increases as the relay ages.

Image is a link to a simulation of the circuit.



FWIW, I think your design constraints as you have outlined in the question are kind of ridiculous. You've pretty much ruled out all the ways there are to drive relays.


An array of h-bridges: Too expensive and not enough room on the pbc board.  


  You can't afford 4 cheap BJTs? How can you afford the relays? You do know you don't need to buy pre-built ICs for this, right?
  Since you really don't need much current gain, you can always build your own h-bridge using two NPN transistors, and two PNP transistors. It sounds like you're only looking at ICs for some reason.
  
  Also, it sounds like you really haven't done much shopping in this area. You can buy low-power integrated H-bridge ICs for ridiculously low prices. For example, the LV8548MC-AH is a dual h-brige driver IC for $1.41 each in single quantities. The MPC17C724 is similar, and is $1.15 in single quantities (though a slightly less easily assembled package).
  Each of these devices could drive two relays.  
  
  If you're really, REALLY price constrained there are even cheaper alternatives out of china, but in that case, why are you using expensive latching relays?

The method shown in the picture below (from Maxim) for every relay driven by two low-side switches:  The relay coil voltage is close to Vcc (3V) which yeilds low R values and a total current draw above what the PS can provide.


  This seems like a really silly way to drive the relays. The only reason I can see for doing this is if you have to use a demultiplexer to drive the relays, and can therefore only set one IO line low at a time.

Capacitor Charge and discharge drive: This method requires large capacitors that are physically too large to fit in the enclosure (defeats the purpose of using single coil relays).


  How small are your relays? I've shopped around a fair bit for latching relays, and I don't think I've seen any that are even similarly sized to reasonably priced large-value low-voltage capacitors.

An array of analog switches (one per coil as an on/off switch and two to flip Vcc and Gnd and reverse pol): This might work, however analog switches with low on-resistance are expensive compared to low-side mosfet or bjt switches.


  This is going to be MUCH more expensive then the H-bridge option, and offer negligible benefits.