Grummer, excuse me for being blunt, but that formula is just by-the-book-dumb. Would you allow an item of continuous True Strike for the same cost? Formulae are mostly meaningless in magic item pricing. And even if you want to come up with a formula, the OP has a much more sensible approach (taking AC and save bonuses first and going by the table).
On to the OP's question: what is the reason your PCs cast Protection from Evil normally? In my group, the players couldn't care less about the low AC and save bonuses - but protection from charm and mind control, now that's a biggie! That effect alone negates a lot of monsters' (and spellcasters') schticks right there!
No risk of mind control at all, ever, that's the reason why somebody would want that Helm. And that's why I think this effect should be taken as the yardstick of said Helm's cost, no matter the fact that it's based on a lowly 1st level spell. We can still add in deflection and resistance bonuses and the summoned creatures thing.
(BTW, the "untouchable by summons" effect may be very situational, but it's extremely good whenever it comes up - full blanket immunity is always a good thing.)
Now what are comparable items?
The Cowl of Warding grants Mind Blank, wich is obviously much more powerful. It's also ridiculously expensive (200,000 gp IIRC), so let's forget this one.
A Periapt of Health grants continual immunity to disease (which is rare, and not that threatening), while the Periapt of Proof Against Poison grants likewise immunity to poison (which can ruin your day at lower levels, and isn't that rare). The Periapt of Health is 7,400 gp, while the Periapt of Proof vs. Poison is 27,000 gp. Can we measure the immunity to charms and mind control against immunities to disease and poison, respectively? I'd say it's much, much closer to poison - maybe even better, since poison usually ceases to be a concern at mid levels, while charms and domination can still turn your friendly neighbourhood Barbarian into a raving axe murderer at high levels.
The Scarab of Protection provides continual immunity to death effects and level drain. It turns to dust after 12 such attacks, however it also continually provides SR 20. It's 38,000 gp. If we say that the 12 charges thing and the granted SR cancel each other out (the SR is actually less useful than the deathward), we're looking at (roughly) 38,000 gp for blanket immunity to death effects and energy drain. This is probably underestimating the immunity granted, but death effects don't occur as often as charm effects, and I hope you don't fight Wraiths/Spectres more often than Mind Flayers/Succubi. So might 38,000 gp be a good idea?
The Horn of Goodness can produce a Magic Circle vs. Evil for 1 hour/day, costs 6,500 gp. We're finally getting somewhere: continuous items are obviously much, much better than command-activated, 1/day items, but Magic Circle is the AoE variant of the spell we're looking at. Still, you could probably make an argument that 24h/day protection should cost 24 times the price of 1h/day protection...
Based on the above ruminations, I still can't really put a price tag on your Helm of Protection from Evil. I believe that protection from charm and mind control alone are probably worth more than 30,000 gp. I haven't even figured in the AC and save bonuses into this. And the very situational immunity to summoned creatures is very hard to estimate as well. Really, I'd recommend just going with the following guidelines:
How highly do you rate complete protection from charm and mind control? If you think it's better than poison immunity (as I do), you're looking at 27,001+ gp already.
How often do your guys fight non-evil creatures? If it's less than, say, a third of the time, don't reduce the costs for AC and save bonuses significantly.
Do you ever plan on unleashing a dedicated summoner on the group? If not, ignore the protection vs. summoned effect when determining the Helm's price.
Hope that helps.