Party, even a $100.00 does have a dramatic influence on the long shots, but in the range you are betting in, it's probably less. It all depends on the pool and the amount added to the pool.
There is a simple formula, used to be available from the TAB to see at a glance how your bet will affect the odds.

The biggest problem I see, is the people who follow price fluctuations, I had a place bet yesterday, when I put it on, the place odd was $2.80, sure I put on a fair amount, so it came down to $2.20. Still not too bad, but this is where the price jockeys come in. The horse finished paying $7.80 for win and $1.70 for the place, it came second or third can't remember I only bet for the place.
The point is, I would not have bet it at under $2.50 let alone $1.70.

In the lower price range it probably doesn't matter that much, I don't know.

You not only have to pick 'em you have to watch out for the hyenas coming in for the scraps of your labour.

Good luck