Question: I'm not a big fan of spending 100ish dollars to buy all three core rulebooks. Are there monster stats to be found online? Also, is there an online substitute for the DMG (for items)?

Answer: 
WotC has released free PDFs that provide you with *some* of the content from the PHB, DMG, and MM:

* [Basic Rules](http://dnd.wizards.com/articles/features/basicrules) - Apart from the core rules information, the Players' PDF gives you the most iconic D&D classes and races, and a small selection of spells. The DM's PDF gives you a small collection of monsters and magic items. They can also be viewed online - [Players](http://dnd.wizards.com/products/tabletop/players-basic-rules) / [DMs](http://dnd.wizards.com/products/tabletop/dm-basic-rules).
* [SRD](http://dnd.wizards.com/articles/features/systems-reference-document-srd) - All the races from the PHB (but only one subrace of each), and all the classes (but only one class option template for each), and a good selection of spells, monsters & magic items. There are also 3rd-party sites that lets you view (and search) the SRD online: [Open5e](http://open5e.com/) or [5e SRD](http://www.5esrd.com/). Also, [D&D Beyond](https://www.dndbeyond.com/sources/basic-rules) combines the Basic Rules and SRD into one, but just calls it Basic Rules.
* [Elemental Evil Player's Companion](http://dnd.wizards.com/articles/features/elementalevil_playerscompanion) - All the spells referenced in the Elemental Evil adventure that weren't in the Basic rules, and a few new races.
* There are also supplements for [Hoard of the Dragon Queen](http://dnd.wizards.com/products/tabletop-games/rpg-products/hoard-dragon-queen), [Princes of the Apocalypse](http://dnd.wizards.com/products/tabletop-games/rpg-products/princes-apocalypse), and [Rise of Tiamat](http://dnd.wizards.com/products/tabletop-games/rpg-products/rise-tiamat) that detail all the spells, monsters, and magic items referenced in those adventures that weren't in the Basic rules.

(I have created [an index of all the above documents](http://www.enworld.org/forum/showthread.php?477146-The-Cheapskate-Index-An-index-to-WotC-s-free-PDFs) that you may find useful)

You may also be interested in the [Unearthed Arcana](http://dnd.wizards.com/search/site/unearthed%2520arcana) articles, that detail not-fully-tested options. These may or may not appear in later books, in modified form.

There are also 3rd-party supplements on the [Dungeonmasters Guild](http://www.dmsguild.com/browse.php?pto=0&pfrom=0), some of which are free.
You can also look to Fantasy Grounds (www.fantasygrounds.net). It is an online campaign manager for D&D, and other game systems. It has the rulebooks built into its libraries, and allows the creation of all the hard cover printed books. You also have options to create your classes/abilities/etc.