Jaxlongtimer and Lakelander hit on a lot of the key points about Brightline's prices, but I'll share a few random thoughts and anecdotes as an occasional Brightliner.
-Brightline is a premium train service. Much like a luxury hotel, they would rather keep prices high and service a higher end clientele and not be full, then lower prices and upset their key end user. They don't want homeless people being able to ride, which does happen on Tri-Rail and other trains. Not agreeing with this in concept, just saying that is the approach they are taking.
-To the above point, they do offer deals and discounts to corporate partners. I recently worked for the the largest company in Florida (by market cap) and we had a standing 25% discount on ticket prices and occasionally got even better deals.
-Prior to Orlando opening, business travelers and upper income people traveling to events throughout South Florida seemed to be the core of their ridership. There are a lot of business people that use it to day trip between Miami, Fort Lauderdale, and West Palm Beach for work meetings, visiting clients, or functions. I once rode down from West Palm Beach to Miami for a night gala with a group of 10 coworkers. We ran into about 20 other people going to the same event on our car alone. I don't know if that same synergy will exist between South Florida and Orlando, but have to imagine there will be some, especially with conferences.
-They offer 25% off tickets for college students. Brightline makes it easier for kids to go to UCF, FAU, etc and not have to have a car to get to and from their hometowns (if near a stop). Parents will probably happily pay the premium prices a couple times a semester if it saves them the expense of car/insurance payments.
-They frequently run specials when demand is low. In summertime when less tourists and residents are in South Florida, they typically have cheaper rates and allow kids to ride for free. I expect them to offer lower rates next summer between Orlando-South Florida.
-As a solo traveler or couple it is often cost effective to ride vs pay for gas and/or parking. My wife and I occasionally Brightline from WPB down to Miami, and uber to Miami Beach for a weekend getaway. It's cheaper or close to a break even to do this and not pay for hotel parking and gas, plus a way more enjoyable trip, as Dade/Broward traffic is bad and getting worse. This will also be useful with direct service to Orlando International.
-Someone once told me that Brightline is not a rail company. It is a real estate development company. Not entirely true, but they are a developer of multi-family and I believe make other JV deals near their stops. So their revenue and business model is not entirely built on ridership and ticket revenue.
-They also make revenue from partnerships with sports teams, cultural attractions, and venues. In South Florida you can ride Brightline and take a special shuttle to Heat, Dolphins, Inter Miami, and Marlins games. In the summer time, the local museums offer free entrance to children with their Brightline tickets and purchase of an adult ticket. Not sure how the revenue works there, but they get something from those arrangements. Even though I think the relationship got strained from the station issues, I still expect Disney and Brightline will work out a partnership.
-There are tons of wealthy Americans, Europeans and South Americans that vacation in South Florida every winter. Brightline will make it a lot easier for some of them to pop up to Orlando and hit some parks up, though I'm sure many have no interest.
-Similarly, tons of foreign travelers hit up Disney throughout the year. As Jaxlongtimer pointed out, taking a train to South Florida is probably a lot more appealing than renting a car in a foreign country.
-Final point is that I don't think Brightline will ever come close to filling its cars until they add more intercity stops and complete the connection to Tampa. They are legally obligated to provide a station in Martin or St. Lucie County in the next five years. Stuart is preferred, but will allow no density, so you will likely see a stop in Fort Pierce, which has the bones for a great downtown. I've also heard there will be at least one stop in Brevard County, but could see them adding two. NextEra/FPL and the City of Palm Beach Gardens are pushing hard for a stop there, where you have a ton of high salary jobs and shopping. If Tampa happens, there would have to be a Lakeland stop. So ultimately, you would have a regional system that looks more like a European system, with express trains and regional trains. I personally don't believe Brightline will survive long-term without becoming a regional system with a wide variety of stops along it's lines. Time will tell.