So, I declare war on these two people (now one), and I start off without about 260 mil. After 3 days of war, I'm sitting at a dismal 227 mil.

Here's what I've done:
Bought 8 CMs total
Done a destroy nukes op, 2 sabotage IRS, and one gather intel
Sent one package of 3 mil, with 2 more pending (which would make it 221 mil)
Was not attacked

I haven't done anything else. What can explain the 30 mil spent then?

It would cost me $11,432,564 to do one Sabotage IRS spy op on that guy... since you did two I am guessing that can account for a large chunk of your $30m spent. The rest is probably just the costs of buying CMs, tanks, etc.

Your opponent is 19 days inactive, and if you examine the description of that spy op in the information index you will see that the number of days inactive of your target is a multiplier for cost of that op. Aimee Mann is quite correct.

Question answered, closing thread.