How many raids do you do in a day?
What difficulty of raid are they?
How many times do you die doing it?
How much synthite/parts do you receive?
How much do you want to get for that amount of effort?
Instead of answering each question individually, I'll just describe my process which should answer your questions. I treat synthite basically the same way I treat money. I have an arbitrary number I pick that I don't like to go below so I feel safe. Previously that number was 10,000 synthite. Since they had the triple synthite weekend, I grinded quite a bit and now that number is 50,000. Anyway, I play enough to earn the synthite I want to buy new outposts or keep my current outposts active. But at the end of the day, I'm generally close to where I started. I've never really counted the amount of synthite that I spend, but it's quite a bit, especially on weekends. On weekdays I generally keep around 3 outposts active on average. On weekends, I keep 5 outposts active and will routinely prestige them early to get new raiders.
As far as what I raid, I generally stick to brutal and dangerous. Once a month or so I might slum it and do some normal missions. I almost exclusively do raids that guarantee synthite rewards. I try to focus on outposts that don't look like they are going to be a time sink and am generally successful in that regard. I'm pretty sure my average raid time is well under 5 minutes. I also don't die that much. Most raids are completed with 2 or less deaths. As far as how much I would like to earn, I would make it so the daily reward for logging in is 1000 synthite and I would double the guaranteed synthite rewards for difficult and brutal. Normal can stay where it is.