The first time we had an auction (in the gym), admission was free. By the time we had our third auction, we were able to charge $25 a head for a really nice adult evening out at a local country club. We netted $6,000 the first year, over $30,000 the third time. Definately experience - and nice auction items - paid off. If you have only modest items, I wouldn't charge admission.
I agree with the suggestion to combine smaller items into a theme item or basket. With a little imagination and a couple of other items, a gift certificate can be the basis for a "girl's night out" bundle. Just be careful you don't overprice the basket if the only thing of real value is the gift cert.
When we had trouble bundling certs, we put them in our raffle. You'll never get more than a $30 bid for a $30 grocery store cert, but if you charge $1 per ticket for the chance to win that cert, you might end up with 40 tickets in the box. We always have about 15 raffle items at our silent auctions for items like this.
Our president has the motto "It is what it is" whenever I get stressed about the difference between our goal and our donations. We have the same problem. $15-$25 items and gift certificates are the majority of what we are getting.
We are also doing a spagetti dinner, program and selling grab bags to try to drum up more money and attendance.
A high school near us charged to get in and only 65 people went. Their choirs are larger that. That would be a very, very small percentage of parents. I wouldn't reccomend charging.
I don't think it would be a good idea to charge a fee being they are going into a room to fork over more money. Make sure you set a minimum bid or you could get gyped....A local golf course donated to all the elementary schools a $500 year membership. We set a min bid of 100 bucks and got 375 out of it......another school didn't set a minimum bid and got 82 out of it. Take your low ticket items and combine them into 1 package to bid on. Like tshirts with hats and a coffee mug or whatnot.......what is not bid on you can give to the teacher appriciation committe to give out to teachers during May
We don't have a fee. The silent auction and sock hop are free. The families can choose to spend money on concessions, raffles, moonwalk, face painting, etc.
I've never been to a silent auction that charged a fee (unless it was in conjunction with something bigger - like included a dinner). But I suppose there are different ways of doing things.
That would be a very, very small percentage of parents. I wouldn't reccomend charging. 
