Get fletching, Firemaking, and you may still have some money to get close to herblore.. I would do that just to get a few 99's instead of just one. Or do smithing, always an epic one to see someone wearing.
and join the 100k+ that have it already?
Back to subject, construction. In the end you'll be happiest because least people have it, and a beautiful house is always good to chill in and have parties. My 120m+ i earned from BH is actually going to be poured into construction.
What I got to say to the other money-costing 99s:
Smithing- could also consider this 99, but the cost is only a bit less and it has less use outside of repairing your barrows for 1/2 price in your house.
herblore- it costs more than construction. and when your pots don't buy the supplies don't buy either most of the time.
summoning- actually the most useful out of all, but the thing is i doubt you have the patience.
Crafting- no use and too many people have 99..
Prayer- actually is quite useful in fights and gwd and worth it, but then a lot of people already have 99.
Ranged- worth it if you like to go armadyl or you're a pker that's still lower than lvl~105 (since above ags and claws will pwn you since most people are 90+ atk., but if you got 99 def and a low cb, go for it!)
Magic- this is so cheap.. like 10m and possibly even a profit? shouldn't be in the list
chinn i guess
