It's pretty simple actually. When they say the bet is "based on $100" that doesn't mean you have to bet a $100, its just showing you the percentage you would win.
For example, if it shows -110 then that means you are picking that team to win and cover whatever point spread it shows with it, but -110 means if you "did" bet $110, you would win $100 if your team listed covers the point spread. You can bet $5 if you want, or $10 if you want, it doesn't matter.
Simple math is just to take 100/110 and that tells you the percentage (roughly 91% is what -110 bet pays). So if you bet $10 and win, you would get paid your $10 back plus the $9 and change (91% of your bet).
If the bet was -115, then thats not a good bet, obviously you want the higher percentage because thats how the casino's make money, so -105 is a much better paying bet. (which that bet is around 95% paid back to you).
If you lose the bet, then you just lose whatever you bet of course and you can't walk up to collect your winnings :-)
Its actually quite simple when you look at it that way.