I (as ~telophase) wrote an Artist Alley Survival Guide a few years back. The site it was hosted on is currently down because it was hacked and I haven't gotten around to restoring it, but the text of the guide is on Google Docs. Reach it here.

I will totally see you there! (First time attending this con, excited! )

From checking out ECCC this year I noticed a couple of trends. First is that your display is important! It's what draws people to you and attracts their attention. Even if you can't afford one of the fancy collapsable vertical banners, or life-size cutouts or whatever, make sure your display is eye catching. Your signage is important - also even if you just put a foam-backed poster on an easel behind your table that will be better than nothing.

As far as products go, I took as many free things as possible - business cards, buttons, keychains - etc. Everyone loves free! And if it has your web information on it, all the better (I did actually followup and check out the websites of most of the business cards I took). To help with the printing costs of everything keep an eye out for those Free-Short-Run business card offers (usually about 200 cards) - if you use a few of those from different companies (usually different sizes, like the really thin cards or the circular ones), and print a different image on each one, suddenly you have variety, even in your free offerings! Various websites offer trial runs like that, so I'd check it out. Also give a call out to local print shops, some will offer discounted runs of things like buttons if you put the printshop's logo on the back too. (I could ask around my school - Cornish - and see if anyone knows any good local offers).

Also on the free side of things, for promoting your webcomic - I always thought it would be great to offer sample pages! Resize them down to four pages per sheet o' paper, then get a set printed and cut. If you did color panels on the one side, and black and white contact information on the other - it wouldn't be too much to print. The beauty of this is that you get four pages for each print! Stack them up on your table one right next to each other for each page (so people can take one or come back for the rest if they are interested) and now people have a take-away-free comic, with information about how to read more!

For prints and commissions - deferring to people who have actually exhibited before. When I was there I only spent money on full copies of books or original art.

One thing I'll say though, at ECCC there was a girl (Katie Cook of KatieCanDraw.Com) who was totally making book with her commissions. She was selling tiny, personalized water color drawings. She had a stack of like 3' × 3' cards of watercolor paper (meaning she could use the cheap stuff - since the cards were so small, they held their shape even being made from a lighter-weight stock) and she was drawing people's pets (also famous characters)! They would hand her their cell-phone or whatever, and she would do a quick simple line drawing with pen, then paint maybe three colors of watercolor, cell shaded on top - sign her name and be done with it. Each commission took her no more than 2 minutes and in that time she talked to the person, promoted her wares and just conversed!

She sold each card for $5 - during the peak hours she probably made 25+ cards an hour (there was always a line behind her booth - during the slow hours she'd make extra of popular characters from movies and generic animals). Compare that to the artists who take three big commissions a day and sell them each for $100. She both made more from her tiny commissions than the bigger artists AND drew more people to her booth AND at the end of the day had more people walk away with her name on a card than anyone else.

Anyway, just my thoughts.

My only experience is with anime conventions, but buttons and small prints are always what sells out first, outside of cheap commissions. I personally would buy Taco Love, your Ludo piece and DEFINITELY go for popular stuff. Which will probably be Avengers, Spiderman (and upcoming comic book movies)

Under 5 bucks for small sketches, $5 for inked, more for colors, and add fees for multiple characters. I've seen people offer headshots/half body/full body prices too

From my personal experience with Artist Alleys, unless you're like gnarly good (like, among the top 2 or 3 artists in the artist alley, and/or are really well known) people aren't too interested in prints. I've had much better luck selling stickers and key chains and bookmarks over prints.
It seems to me like cons are mini vacations for a lot of people, and they really like to pick up souvenir type things while they're there. - Something to think about!

I also recommend creating as much new art made specifically for the con as you can, because when I've tried to appropriate past work to sell at a con it has never gone over well.