It depends what you mean by "affordable". If you want an accredited degree it's going to cost you.
One free option is
Nations University ... they are free (unless you live in the USA, even then it's only $100/year). They are NOT accredited but they are becoming more and more respected, and they are applicants for accreditation with the DETC (which is not the most "prestigious" accreditation but it's legal and valid in the USA). If you need or want an accredited degree this won't work for you, but if not this could be a good option.
If you just want to learn more and aren't concerned with earning an associate or bachelor's degree, you could try
Gordon-Conwell Theological Seminary's free online certificate program. It's entirely free, and they will send you an official certificate from the seminary if you complete the program. It's not the same as a bachelor's degree of course, and you can't transfer any of the work into a degree program.
Besides that, a (relatively) cheap place to earn a regionally accredited associate or bachelor's degree (regional accreditation is the best type a school in the USA can have) is
Clear Creek Baptist Bible College. They are also accredited by ABHE and offer several online degrees for $182/credit. (For comparison, the cheapest you'll find at most other similarly accredited schools is $300/credit.) I know nothing about them other than what it says on their website though. Probably the most well known option is
Liberty University, their Bachelor of Science in Religion is $295/credit.
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What are your future goals? Pastor? Other ministry work? Or is this for your own spiritual growth without any such goal?