Get two Juniper SRX100Bs and configure them with an IPSec tunnel between.
 
Cost: ~$1100 + labor
Time: should take an experienced person less than 2 hours
 
It'll operate as fast as your network connections and behave as though the two networks are one.
 
Or, if you want to go even cheaper, two Cisco 2621XM routers used on ebay. Timeframe will be the same, but the cost of hardware will be a lot less. You won't be able to break 10mbps IPSec throughput going this route, but that should be adequate for most Internet connections. My recommendation would be the Junipers.
 
Or, if you want Layer 2 adjacency, as if the systems were physically connected to the same LAN, two Juniper SRX100Bs configured with CCCoMPLSoGREoIPSec.  You'll get about 10mbps throughput on them, but you'll have true L2 adjacency across the wire.  This would take a very experienced network engineer to put together and would take roughly the same amount of time as a regular IPsec tunnel and hardware costs would be the same.