First of all, bear in mind the odds of getting a 5* hero from summons is at around (estimated) 3%. Second, even if you get a 5, you may not get Guin, so the odds for her, specifically, are really (less than 1%) low.
Second, only spend money if you really can afford it. If this money won’t make a difference in your life, in your accounts, in your food and in your time with your family. It’s a lot of money to spend if you don’t plan, and no amount of pixels are worth it if it’s going to be missing from the things you must do.
That said, if you are still ok with the odds and the money won’t make a difference for you, go ahead and get ready to spend a ton. You may get her in the first pull, you may do 10 times 10x pulls and never see her, so other people’s experiences can’t help you. The odds are not spread out, if 10 people got her at once, it doesn’t mean you will, or that your odds of getting her at once will be lower.
This is the nature of Gacha. If you want to understand a little more, please read here:
I’m not here to say you can’t spend, I’m just here to help you make an informed decision on your spending.
If you still want to go ahead with spending on the banner, I’d suggest setting up a budget, like you did, with $280, or $300, or whatever, buy all the gems at once when the Knights of Avalon banner is on (wait until then, because there’ll probably be a gem offer) and then do the pulls. If you don’t get her after spending all the gems you bought, if you reached your budget, stop pulling and leave with whatever you get. There’s always next time.
Hope it helps. May the odds be on your favor.