Hi Slimfish,
The circuit is from Microchip and Fairchild: “Li-Ion System Power Path Management Reference Design”, document # DS51746A, 2008, pages 5 & 12. The FET – Diode combination was made by Fairchild for this specific application (disconnect battery when external power is present).
The cost for both ICs: FDFMA2P853 and MCP73832 are $0.64 USD at DigiKey.com. For reference, the LTC4054 is $1.82.
I am an older analog hardware electrical engineer, who is always amazed by the fantastic things microcontroller designers and programmers can accomplish. Also amazing is the difficulty they have with analog circuits. I am willing to help Seeedstudio with the DSO Nano, since it is a fantastic product and they are generous with Open Source activities.
Shazam