which tank are you planning to dose?? whats your lighting?? are you using co2??
You can go with liquid ferts (seachem line- Flourish, Iron, Nitrogen, Potassium, Phosphorus) or you can go the dry route (rex's dry ferts)
If you get liquid then it will cost you around $4-5 per bottle x 5 + s/h = $27-32 (if ordered online)
With dry it will be around $21.05 if you get the combination pack
both are just as good.. dry are cheaper in the long run.. if you are dosing a tank like a 10g-20g then it would be fine to start with liquid, get used to using ferts and then switch to dry
word of caution. it depends on what plants u have and the load of plants. also the wattage because if ur plants arn't going to eat up all the ferts u add, it will just cause algae.
i find that dry ferts are better with high light tanks with co2.