[Runescape] Wasting money?

Now I'm not a rich person on RS. I've probably got a total value of 10m (probably less, read on)

Anyway. I've been looking at what methods make money...and the method that makes about 24k for a 1min trip, is making house tele tabs

So. Earlier today I had a look at how to make these. Mahogany Eagle Lectern...67 Construction required. I had 50.
Well, I did a bit of adding up, worked out all the costs, and found...ITS WORTH IT

Costs:
7.5k Oak Planks (700gp each)
2 Mahogany Planks (2k each)
1 Gold Leaf (135k)
Tablet runes: 380gp per spell
Soft Clay: N/a

Profit:
House Tele Tab: 1350gp each


Within time, you can see this will make profit



Now. The reason for the topic title...
I was talking in my clan about me doing construction. And all of them were like WHY?!?!? ITS A WASTE OF MONEY!!!

So, do you think it's a waste?
What do you do when you get some money?

PS: Construction level now = 60 (7 to go)