Most popular auctions with expiring domains are at godaddy, namejet, snapnames. Search for expired domains name its very easy if you have domains list. With a bit luck, you can drop good one with PR5 cheaper like $200, just need to know which domain have not fake pr. List of domains are not free, but updated dailly with aprox. 50.000 new expiring domain names.
What type of business are you considering of building something on using the domain name?Is it something you believe in and willing to put your all, how much can the monetize with the website, etc..Ask yourself much questions before proceeding to finally selling it.Also, it doesn’t matter if a different company is using a different extension.It means you have upper hand to get their traffic if you build something using the .com.Good luck
That said, my recommended budget for beginners would be $500+. Using this budget you could buy a bunch of high-potential $10 domain names, expired/dropped domain names or a mix of both. It’s very important to invest only what you can afford to lose and treat this as a side hustle till you get the momentum going. As you gain experience, industry expertise and some sales under your belt, you can then consider slowly growing your business into a full-time gig.