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.
[D&D Beyond](https://www.dndbeyond.com/) is the official digital source.
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For free you can access the [Basic Rules](https://www.dndbeyond.com/sources/basic-rules); this includes [races](https://www.dndbeyond.com/sources/basic-rules/races), [classes](https://www.dndbeyond.com/sources/basic-rules/classes), archetypes, [monsters](https://www.dndbeyond.com/sources/basic-rules/monsters), Unearthed Arcana, etc, everything you need to run a game. The site also contains useful features, such as character builders, and campaign organisation tools. Everything is searchable, linkable, and well organised.

I would go so far as to say that most players probably use D&D Beyond for its tools and rules referencing, even if they own the books in some other form.

Anything else that costs money, will still cost money to access.
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But, D&D Beyond does make it convenient to buy additional content. [You can buy digital copies of rules, supplements, and adventures](https://www.dndbeyond.com/sources) one at a time or in packs too.

Most items, classes, and monsters can be purchased individually at an even lower rate. Say you want the Abyssal Chicken, instead of buying the source book it appears in ([Baldur's Gate: Descent into Avernus](https://www.dndbeyond.com/marketplace/source/baldurs-gate-descent-into-avernus)) for $30, you can buy all 38 monsters in that book for $5, or just the chicken for $2.

Note that the Basic Rules include the SRD
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The Basic Rules contain the entire SRD, so there is no need to worry about it:

> 
> This section contains the Dungeons & Dragons Basic Rules and the rules in the SRD, released as part of the Open Gaming License.
> 
> 
>