There are many ways to figure your fee, but the bottom line is that you need to receive a reasonable hourly rate for your services and you need to be efficient with that service.  For structural design services, you should be somewhere between 0.75 percent and 1.25 percent of the expected construction cost (total construction), depending upon the complexity of the design.  Another way to figure is to compute your fee based on the number of plan sheets you will produce.  In US dollars, that could be anywhere between $1500 per sheet to $3000 per sheet or even more in specialized cases requiring many details.

If you are considering an hourly rate, remember that you will need about 3 to 4 times the salary rate you expect in order to cover your costs (downtime, non-reimbursed expenses, insurance, etc.), so if you want to pay yourself $25 per hour, charge $75 to $100 per hour to cover everything.  If your costs are low (working out of your house, etc.) you can get by on a salary multiplier of 2 or 3 times the expected salary rate.  Set your hourly rate at something close to the rate charged by other similar practictioners in your area.  This allows all to make a reasonable fee, keeps you in line with the local "Standard of Care" argument for liability purposes, and keeps peace among the locals.

I am assuming you are properly licensed to do the work.  If not, don't attempt the work.

Just kidding. The prices are very atractive in your country. Here in Brazil things are very different.
For an installation of an indutrial plant, the prices for a complete sructural design, with foundation location and detailing sheets(02), reinforced concrete sheets for colums(4), beams(5), slabs detailing sheet(02) and trusses detailing sheet(3) costs about US$ 3.000,00 (R$ 6.000,00).
Thanks a lot

I can talk about the Argentina's fees.
These are the prices (without taxes) for a regular building of eigth plants (total 1000 m2) :
-Structural proyect and Calculum : aprox. u$s 1 /m2
-All the structural details (CAD plans, sheets, specifications, etc.)and technical assesment during the execution : aprox. u$s 1.6 /m2
Cheers and Happy New year
Herny