You have to consider overall content. A better comparison would be scenarios:
Civ 4: American Revolution, $50
Warlords: 8 scenarios, $30
BtS: 11 scenarios, $30
Total: $110, 20 scenarios = $5.50 per scenario
I'm assuming that no one's buying individual maps here:
Civ V: Mongolia scenario, $50
Civ V DLC (with civs): $20 (Denmark+Explorer's Pack, Inca+Spain, Polynesia)
Total: $70, 5 releases = $14 per release
Civ V DLC (without civs): $10 (separate Denmark, separate Explorer's Pack, Inca+Spain, Polynesia)
Total: $62.50, 5 releases = $12.50 per release
Not as good a selling point, especially when you consider that BtS's scenarios had many more gameplay changes.
Another way to look at it is by pure product:
Civ 4: 34 civs + 20 scenarios = 54 products
$110 / 54 = $2.04 per product
Civ V: 24 civs + 5 scenarios = 29 products
We have to add Babylon, so add another $5 to make it $75.
$75 / 29 = $2.59 per product
A 0.51 difference. Not too terrible considering that you're paying taxes at Wal-Mart.
Consider also that Civ V civs are programmed with more core gameplay changes, as opposed to Civ 4's simple trait distribution. What's still the best selling point is that you can buy this DLC over time, as opposed to all at once. You're not pressured to play everything at once because you shelled out enough dough (that reminds me, I need to play everything in my Valve pack...).