Hey, same problem as you.
As said, I am running PhosGuard on a similar size tank as yours (200g, 160g volume of water), and it is just not cost effective for that size of tank. If your phosphates are under .15~ or so, I would think PhosGuard would work. But anything higher, Phosphat-E is definitely (at least for me) the way to go. Buy the smallest amount - it'll last you probably a year. I went from 1.00 to 0.5 in 1.5 months (0.1 a week reduction = 0.6 total reduction, 0.1 from probably feeding, salt mix, or phosphate leech from rock).
The smallest bottle is dirt cheap at $20. Problem is you would also need an IV dripper (amazon, $10 for mine I think), and 1 or 5 micron socks.
You should read into using LaCl (which is Phosphat-E). What I got from reading tons of forums is:
1. Spread out dosage throughout the day (hence, IV bag)
2. Reduce maximum 0.1 a week, or if you can, 0.02 a day.
3. Must use 1 or 5 micron sock.
4. Stop immediately if you see your fish struggling to breathe.
5. Your Alk will drop, make sure to test.
