I did a big thread on pirating #bigfinish (I don't agree with it but I sympathise with those who can't afford it), and I listed some places to make the audios more affordable. So here they are, linked and collated in one thread.

Big finishes site: https://www.bigfinish.com/collections/v/big-finish-for-free
Theres a free section with shorter stories, extracts and some full stuff.
Also check https://www.bigfinish.com/news  and follow @bigfinish on twitter for news about sales, there are usually a few a month at least.

Cheap BF from BF: The first 100 main range https://www.bigfinish.com/ranges/v/monthly-series, and short trips are £3 each https://www.bigfinish.com/ranges/v/doctor-who---short-trips

Some of the earlier stuff like the unbound audios are also cheap on download (£3) https://www.bigfinish.com/ranges/v/doctor-who---unbound

The big finish book club discounts one audio a month to £2.99 https://www.bigfinish.com/collections/v/the-big-finish-book-club (This month its Torchwood!!).

External sources: Spotify https://open.spotify.com/user/doctorwhoofficial
Spotify has the 1st 50 main range audios, the first series of 4th and 8th doctor adventures, and the lost stories. Plus whole boxsets: jago and lightfoot, Iris Wildthyme, Bernice Summerfield, Unit dominion. Over 150 hours of audio

Spotify is free with adverts, or you can get premium without ads for £9.99 a month for an individual or £4.99 for a student. There are family plans (cough cough which you could share between 6 friends instead cough cough) for £14.99. Also theres often deals - eg 3 months for £1

Audible: https://www.audible.co.uk/ep/memberbenefits
£8 a month and you can choose one audiobook, price irrelevant, each month to keep forever. They have a lot of big finish including new series boxsets. plus there are offers - a few months ago the tenth doctor adventures volume 1 was £2.99

Ebay! https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=big+finish&_sacat=0 sort of goes without saying.

Amazon! https://www.amazon.co.uk/s?k=big+finish&ref=nb_sb_noss_1 often keeps the CDs at pre-order prices or below, so again now very cheap but often cheapER.

Humble bundle! https://www.humblebundle.com/ 
A brilliant company that runs bundles of content (often videogames) with a lot going to charity. the first tier of content is always $1. Support Big finish and charities! Big finish often run a doctor who bundle around christmas/new years.

Libraries! Yes they still exist, and are still excellent!
Lots of UK libraries have big finish, and theres an app called borrowbox that lets you with draw them digitally! https://www.borrowbox.com/ 
More free audios!!

If I've missed any off please let me know and I'll add them. Also, if you really just want stuff to listen to, then try podcasts. They are free, and there are millions of them.
Some by Big Finish writers (eg @verbatimfoley 's North West Footwear Database)