I was already mostly done with my research when the other answers posted (thanks!) and they suggest quantities that match mine, but I don't want to lose the extra information I gained, so here's the results of a survey of ice cream recipes.
I looked at the first page of results on Google searching for "vanilla ice cream" and recorded 3 non-custard recipes, and 6 custard recipes. I recorded the liquids, sugar, extract, salt, and egg yolks (in the case of custard) and found the following:
Sugar
The range was fairly large, going from 1 cup sugar per 6 cups liquid on the low end to 1 cup sugar per 3 cups liquid on the high end. The average was right around 1 cup sugar to 4 1/4 cups liquid.
Vanilla
This was a little more variable. On the low end some recipes called for 1 teaspoon vanilla extract per 4 cups liquid, and on the high end some called for 1 teaspoon per cup of liquid. The average here was 1 teaspoon per 2 cups liquid.
Interestingly, unlike sugar, custards and non-custards differed a little in the amount of vanilla used, with custards using less. Non-custards ranged from 1 teaspoon per 1-4 cups of liquid, averaging to 1 teaspoon per 1.5 cups liquid. Custards ranged from 1 teaspoon vanilla extract for every 1 1/2 to 3 cups, averaging to 1 teaspoon per 2 1/4 cups.
Salt
When it was included, it ranged from 1/8 teaspoon per cup liquid on the high end to 1/8 teaspoon per 13 cups liquid. The average is 1/8 teaspoon per 3 1/4 cups liquid but 1/3 of the recipes didn't include it at all.
Egg Yolks
For the custards the egg yolk recommendations varied widely, from two egg yolks per cup of liquid down to one egg yolk per 3 cups liquid. It looks like there's a lot of opportunity for one to choose a ratio that suits their taste and texture desires. The average was one egg per 3/4 cup liquid.
For 9 cups of liquid (about 1/2 gallon prior to churning)
- Sugar: between 1 1/2 cups and 2 3/4 cups. Average is 2 1/8 cups.
- Vanilla: between 2 1/4 teaspoons and 3 tablespoons. Average is 1 1/2 tablespoons (4 3/4 teaspoons)
- Salt: if included, between a pinch (1/16 teaspoon) and 1 teaspoon. Average is 1/4 teaspoon.
- Egg Yolks (custard): between 3 and 18 eggs. 11 is average.