[Lil' Lavery]

This is a decision that really needs to be made by your team rather than anyone else. But that decision needs to be a fully educated and discussed one, especially considering the possibility of registering two teams. There are a number of options that should be considered. Obvious choices include spending the money to create two teams, register the existing team for a second event, or spending it on the materials for the robot.
But other solutions do exist, some of which apply to you much more than others. One is spending the money to purchase materials to create a second bot identical to (part or whole of) your competition robot to use for practice, demonstration, and driver training purposes (particularly after you shipped your competition bot). This "practice bot" could even be built by the girls alone, if desired.
Another option would to be to spend the money on tools, equipment, and other attributes that will help the team for years to come. Buy some machinery that you were lacking, or duplicates of the tools that were frequently used (so two people can use them at once). If you use the money in this fashion it will continue to help your team for years to come as you can still use the tools for several years (hopefully).
Buy large quantities of stock material that you use frequently (although given your $150 budget last year, you might not have any of that yet). In other words, buy your stock aluminum, steel, polycarb, pvc, etc. that you use frequently ahead of time so you already have it on hand when you need it. If you have the storage space, you can purchase for several years in advance.
Spend the money on materials/registration for a second competition. You don't have to solely compete in FRC, you can also participate in Botball, FTC, BEST, or another competition as well.
Buy a trailer to move your robot(s) and other materials in.
Have the team subsidize some of the travel costs from the students. Have the team pay for the lodging and/or transportation costs for any further away events you may opt to goto.
Buy materials, tools, and display options to improve your pit.
These are only some of the many options possible. And some of these would help your situation far more than others, but all of them are possibilities to keep mind as you progress through the years. You could even invest in one option this year, and chose another next year (such as buying tools this year and starting a 2nd team next year).
I would advise you to be careful about starting another team though. Make sure all 21 members are fully committed before looking into that as a serious option. I know it is the case on many teams where tons of students show interest at the beginning of the year, but half or more have vanished by the time build season gets intense. And I highly doubt only a handful of students could support two teams.