I don't feel like calculating, but all the mutual funds I hold are Vanguard funds, and the one with the highest ER among them is Vanguard Inflation-Protected Securities, VIPSX, 0.20%, so my average is less than 0.20%. I think that's about as much as I need to know.
Morningstar usually starts to call expense ratios "low" long before you get down to 0.50% (they don't use a cutoff, they use some kind of percentile criterion for each fund category). Most of the horror-story stuff about the fund company taking a third of your earnings applies to "average" expense ratios in the 1.0-1.5% range. Similarly, although I've seen results that show that expense ratios are the only reliable predictors of performance, what those results show--can't find the actual numbers now--are that the really high expenses, like 1.5% or more, reliably predict underperformance; not that funds with 0.10% expenses reliably outperform those with 0.20% expenses. (There is one important exception to that, of course--in Vanguard's fund family, Admiral shares do reliably outperform Investor shares by just about the amount you'd calculate, given the ER difference).
I don't approve of expense-ratio absolutism. Do the math. Of course you will be richer if you pick up pennies from the sidewalk than if you don't, but what counts is differences in expense ratios, not multiples. A fund with 0.20% expenses may be costing you "twice as much" as one with 0.10% expense, but on a $10,000 investment it is the difference between $20 and $10 and it is ten bucks. I feel that once you get down to 0.25% or lower, it gets to be more about bragging rights and "the principle of the thing" than it is about dollars.
I did have my Aha! moment, though, and it happened in this forum. I had been hanging on to Pax World Balanced Fund, a rather small holding--more than a token, but not a big percentage of my portfolio. It is an actively managed fund, and at that time, it had an expense ratio of about 1%. I justified it by noting that its past performance had been slightly higher than Vanguard Balanced Index, although it was obvious from its holdings why that was--more risk. Nothing extreme or unreasonable, good stuff, international, weighted toward midcaps. But some mischievous poster suggested calculating the absolute dollar number of expenses for each fund we held, and when I did, I was surprised to find that this small holding was costing me more total dollars than all of my Vanguard fund combined. I didn't do anything about it right away, but it ate at me, and eventually I dumped it.
Annual income twenty pounds, annual expenditure nineteen nineteen and six, result happiness; Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.