If I buy a subclass, do I then have to individually buy every spell that it has access to separately? I don't really want to buy a full book right now, or 30 dollars worth of spells.

Would it be easier to just homebrew a similar subclass+spells in? 

You would need to buy any spells that aren't available for free through the Basic Rules (here's a list). You can buy spells individually, or as a subset purchase within a book, so for a given book it'd still be cheaper than buying the whole book. It'll depend on how many spells you think you need. And homebrewing is definitely an option as well. You can recreate actual spells, not just make similar ones, you just can't publish them for others to use. So if you have the print version of a book and don't want to pay a second time, that's the way to go.

Thank you for the reply. I don't think this clearly answered my question though;

If a subclass has certain spells that aren't part of the free content, do I have to individually buy the spells on top of the subclass? 

Hello there Calamina,

If the subclass has spell/s that are not part of the Basic rules, you will have to purchase it/them.

Thank you for answering! Wow that's so unfortunate. I wish they made things like this more transparent in the marketplace. You saved me a lot of money, thanks a million!!

I think home brewing isn't "easier".  It would cost less money, but then it comes down to how valuable your time is or how you value the experience of homebrewing through the tools here on DDB.  (I enjoy homebrewig, but only so many hours in the day). One thing you may be missing about the marketplace is that while content can be bought individually, cheaper than the book, categories of content from within one book can also be bought cheaper than the book.  For example, the spells in the Player's Handbook that aren't in the SRD.  Like all spells on DDB, they're $1.99.  However, $4.99 gets you all 57 spells on that list.  All the sourcebooks are under similar price point bundling, so some books have more "bang for your buck" but the short of it is if you could get the entire PHB's mechanical content for about $10 less than the DDB PHB if you wanted to buy through that method.  

Of course, your subclass may have spells spread over a few books, but being aware of the in book bundles (as opposed to bundles of books) may help your cost analysis.

Quote from ThePatchWorkHoax >>

However I am not sure how this would interact with something like Tasha's optional "Extra Spells" feature which you have to enable.

The "expanded spell lists" would require the character owner to have access to Tasha's "optional class features" which is a mechanical component that only comes with the purchase of the book Tasha's, and aren't available piecemeal to my knowledge.

Remember, any parts of a book that you buy get subtracted from the price of purchasing the entire book: if the whole book costs $29.99 and you spend $4.99 buying all the spells from it, if you decide to buy the entire book at some point in the future it will only cost you $25.