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: 
There are a few things. 

* The basic rules for [players](http://media.wizards.com/2015/downloads/dnd/BasicRules_Playerv3.4.pdf) and [Dungeon Masters](http://media.wizards.com/2015/downloads/dnd/DMBasicRules.pdf) provide all the information needed to start play, but not much in the way of monster stats.
* The [Systems Reference Document](http://media.wizards.com/2016/downloads/SRD-OGL_V1.1.pdf) ("the SRD") includes a subset of the monsters from the Monster Manual and the magic items in the Dungeon Master's Guide. All of the information from the SRD can be found (and searched) on [open5e](http://open5e.com/), which may be more convenient than a monolithic PDF.
[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.
> 
> 
>