You have nearly 700 litres
I imagine you're putting like half a bottle of excel in per dose at that size! It was the same reason I went for CO2 as well, the refills are only £10 from Andy's and I only need to go once every 3/4 months apparently. I use my Excel for my smaller tanks now only.
e i dry ferts is they way to go , for cheapness , if you go down the pressurised co2 route you will still need to dose ferts and trace ,plus you will need to distribute the co2 evenly around your tank , this will meen one or more power heads for your size tank to eliviate dead spots . growth will be amazeing but it takes a lot of work , ive had a 4ft hi tech running for a year now and the results are brilliant but a lot of heartache on the way , its not just as easy as getting a fe and pumping co2 .