Firstly, know that there are ranges of budgets to create buyer personas depedning on if you go with an agency or do them in-house. I’ve worked for a company where we paid $100K to an agency to make buyer personas, and another one where I had to figure it out on a shoestring budget (< $1000). I honestly had so much more fun and learned so much more doing it the scrappy way! What did I use my scrappy budget for? I used budget for incentives to get clients, prospects, and churned customers to talk to me. I developed an interview script and trained others on HOW to interview so we quickly got 30+ customer conversations (recorded, too!). I used a little more budget to run an email raffle for customers that would take a survey. The chance of a $250 Amazon gift card gets people to answer your questions!
But to answer your question directly, if you do not clearly know who your customers are - there is no way you’re being effective with your product development or marketing budget. One thing you might try is to ask 5-10 people internally in different orgs who they think your customers are. Chances are - the answers will be all over the place. Package this up and send it to your boss, or your exec team, wherever you have the best influence - and I bet you can make the case to get the budget and time resources you need.