If you want to spend on yourself, here’s what I suggest you spend on: A good bed, nice chair and healthy food.
(Ha! I bet you didn’t see that coming!)
This image would explain everything.
My list of “things to spend on to invest in myself” is much longer. Those were the top 3.
I amortize costs over the lifetime of the product. I often say, if I spend X amount on this and it lasts me Y (months/years), then I’m just paying Z per (minute/day). Sometimes, I say, if I spend X amount on this and I use this Y number of times, then I’m just paying Z per use when applicable. This way I can compute costs on a daily/monthly/basis.
I’m a believer of YAGNI so I don’t really buy the latest and the greatest (read: most expensive) unless I need them. I fall right in the middle/upper-middle most times. Just enough for my needs, and some bells and whistles.
My longer list with approximate cost ranges is as follows: (All figures in Philippine Pesos)
- Good bed, read: mattress (3k to 9k over 3 years or 2.75 to 8.25 per day)
- Nice chair (2k to 6k over 2 years or 2.75 to 8.25 per day)
- Healthy food (200 to 300 per day)
- Flattering clothes (15k to 20k over 1 year or 41.00 to 55.00 per day)
- Fluffy pillows (500 to 1k over 1 year or 1.50 to 3.00 per day)
- Decent Keyboard & Mouse (1k to 2k over 1 year or 3.00 to 6.00 per day)
- Good enough desktop/laptop (20k to 50k over 3 years or 18.50 to 45.75 per day)
I’ve prioritized the list using these reasons:
- Bed: The effects of quality sleep affects your every waking hour.
- Chair: Good posture is a sort of “prevention is better than cure” thing. More short term, it helps alleviate building up of stress which would affect your quality of work. This also contributes to quality of sleep.
- Food: It keeps you alive. Why not spend on it?
- Clothes: Find the kind of person that you want to be, and dress like him/her. Imitation is the highest form of flattery or Dress for the kind of job you want, not for the job that you have. The clothes make the man. Or it doesn’t. This deserves its own post.
- Pillows: Goes with the bed.
- KB&M: My hands are my bread & butter. RSI and CTS can be literal career-ending injuries.
- Workstation: I spend at least 8 hours on this thing. I would want to have enough for what I need.
That comes out to 269.5 to 426.25 per day or 8,085.00 to 12,787.5 per month.  Factor in bills, rent and other consumables and you’re looking at close to 15,000 to 20,000 per month to live a comfortable life. 
I’m at this point in my life wherein I’m content, materially, with everything I have so far. The only significant purchases I have left are a laptop replacement, a vehicle and a house. The last 2 are just luxuries with my current lifestyle and wouldn’t probably buy them for myself anytime soon (i.e. in the next 3 years).
Optimizing for happiness
Happiness is having everything that you need in the world. The first step to being happy is finding out the minimum that you need. It’s entirely subjective.
The next step to happiness is optimizing the these things that contribute to your daily well being. If you feel well enough on a daily basis, then I think you’re 80% there to general happiness.  The rest is tougher to crack: healthy relationships, sense of purpose, et al. But it sure becomes easier once you’ve got a solid base.For the low low price of Php 20,000 a month, I can pursue the rest of my 20%.
Am I happy? Currently, yes. But it’s a process.
 I actually have another round of computation, wherein the costs are weighted based on how much I use the thing. I’m being lazy so I didn’t include it.
 This is for a single, no dependents person. For a family that is sharing a lot of the things above including rent and utilities, this would be significantly less per person. But of course, you’d have to multiply per head so it comes out larger in lump sum.
A totally pulled-out-of-my-ass formula is : x = number of people, amount = x * 20,000 * (0.9 ^ x). This yields:
[3 ]Pareto principle, the 80-20 rule. Generally speaking, 80% of output, comes from 20% of input.