The last price the 5830s was available at was $130, not $110. Using that figure the ratio comes out to 1.445 for the 5830 rig, so the 5870 rig is actually more economical.
Not that I would dream of buying new rigs right now!
I went with the best possible conditions to show the cheapest scenario. Also just because its not in stock now doesn't mean its gone forever at 110 I mean I just bought a 5830 last week at 110 on newegg.