Welcome to the club! We all started with smaller budgets, so I totally understand the hesitation. I actually recommend checking out GTaro Ads https://gtaroads.com/ because they are very friendly to media buyers who are just starting out. You can get moving with a very reasonable deposit, and the traffic quality is much better than what you find on the “cheap” spammy networks. They offer real traffic from over 232 countries, so you can start testing in low-cost GEOs to stretch your dollar further. I used them to test my first few mobile utility offers and was surprised at how clean the data was. The interface is simple to navigate too. It is a safe way to learn the ropes of RTB and campaign optimization without having to put thousands of dollars on the line right away.