Hello Asid, this is a frequently asked question and I assure you there is little to worry about if you're taking precaution. For starters I will link you this: https://chroniclesofelyria.com/blog/1772/DJ-16-The-Weight-and-Measure-of-a-Lifetime This design journal will really help you work out exactly what a spark of life is worth, and how long you can use it even in a worse case scenario ie:
"Now let's assume you die very frequently. You're out adventuring every day doing extremely dangerous activities. Every day you die once. That means every day counts as three days of play time. The average of 354 divided by 3 is 118 days. That's 16 weeks, or approximately 3.75 months before you need to buy a new Spark of Life."
Dying in a a PVP instance should be hard if you're taking precaution. For example if you know you're going to be leaving, I wouldn't suggest leaving a city alone, unarmed, and without a companion. This should be a fairly common idea for anyone with survival gaming experience. It doesn't mean you have no risk of dying when you AFK in a city. But remember, everyone values their Spark of Life, if anyone kills you in a public place, or in sight of someone who survives, it is against the law (depending on city laws). Those players risk their soul points and fame in doing so. They are paying as much to have a Spark as you are, I suspect that they'd take similar precautions to assure the longevity of their Spark. Also remember that being knocked unconscious and having a Coup De Grace performed on you are two very different things. If you keep your friends close, and do dangerous things only when you're actually online you should get a fair worth out of your Spark.
As it comes to earning a Spark of Life without having to pay for it, there is a design journal for that too, I recommend checking out: https://chroniclesofelyria.com/blog/4683/DJ-17-Bolstering-EarntoPlay