Having trouble with a backdoor roth on form 8606

I'm finding conflicting information on the internet so I'll ask here. First some background:  

1. My wife is a high income earner and has a 401k at work. She has no IRAs except for $5500 that we put in a Trad non deduc ira and then immediately converted to a Roth. no gains, no losses.

2. I am a parent and now carry a $13,700 rollover IRA from previous employer as my only other IRA. It has grown to $16,500 due to investments. A spousal contribution of $5500 was put into a Trad non deduc ira and then immediately converted to a Roth also.

The problem I'm having is with line 2 of our 8606's. I'm not sure what the cost basis of each should be as the $5500 was already taxed previously and isn't in the Trad IRA account any longer.

Thanks to all for any assistance !