Throw some more until it does work. There's no guarantee that spending money will work but if you get a good manager with a proven track record and give him the money he needs, it's the most likely way to get success.
The 'happy medium' compromise is often the worst option as you spend money but with less likelihood of success. Over two or three seasons of almost doing it but not quite, the money starts to add up.
If you spend a bit more than your rivals, the benefit can be negligible. You could spend a bit more than your rivals for three seasons and, due to a bit of bad luck or a misjudgement, still not get promoted.
If I owned a club I could inject say £5m a year into it and, because some of my rival owners are putting a bit into their clubs, find I only have a wage bill slightly higher than theirs. I could spend four seasons in that situation and not go up.
Or, I can chuck £10m a year into the club and blow the rivals out of the water knowing that, providing the manager and those responsible for recruitment aren't massive knobs, if it doesn't work the first season it'll probably work the following season.
And if I'm not prepared to take the hit, I shouldn't be the owner of a football club.
Either run the club on a tight budget, get an agricultural manager like Grayson and boot your way out of the league or committ to buying your way out of the league, the middle ground offers the least.