Welcome to the "LPS Club", one purchase is all it takes before you're officially sucked in!
There is such a wide variety of packages to get started, everything from single pets to tubes of several pets to playhouses and carrying cases!
One great way to start would be to get the Little Lovin' Playhouse (retails for about $19.xx) which comes with a nice sized house and a handful of pets. They also have sets around $12.99 or so that include a small playhouse and 2-3 pets. The single or pair of pets retail for anywhere between $2.99-$6.99 depending on where you shop (on average, it is about $3.XX-$4.XX) so if she wants an additional specific animal, you could get a pack or so of those to go along with it.
You can find these at Family Dollar, Big Lots, Target, Wal-Mart, K-Mart,
Sams Club, Kohls, Limited Too, Toys R Us, etc., so depending on where you shop, you can find lots of different deals and various playsets that would start her with a few pets and a playhouse for her to play with them in.
Good luck and happy holidays!