Monthly transfer is the wrong starting point, because a port is a rate. Size from peak concurrency, per-unit bitrate and headroom — then check the cost per gigabit of each tier, because that curve is not monotonic and the cheapest gigabit is not where most people buy.
The BGP session is the easy step. What decides whether your route reaches anyone is the LOA, the IRR objects, and a ROA whose max length is not quietly authorising somebody else to originate your more-specifics.
The machine is the easy part. Address reputation, geolocation and an abuse process that answers decide whether the service works at all — then conntrack, MSS clamping and IPv6 decide whether it scales past one node.
Transit buys reach to everywhere by the longest road. Peering buys a short path to the networks you actually send to. What each one costs, how BGP picks, and how to read the difference from your own shell.