HRT should be taken every day. It is a lifelong protocol and is not cycled. The dose is also just high enough to bring your testosterone levels back into the normal range. For example, I would take 100mg/wk for my HRT dose, but a full muscle/strength building cycle would be 500-1000mg/wk for me.
If you were to take the HRT meds (currently gels for you) only on workout days, your testosterone levels would vary from day to day and you will feel like shit, mentally and physically.
As far as cost of injectables vs. gels, that depends on your insurance. The gels will cost whatever a monthly prescription typically costs, which for me used to be $20/mo. The injections will cost more if you get them done at the doctor's office as you'll likely have to pay for the visit, and for me they had a flat fee for the injection (irregardless of dose I think) which was like $15 (not sure if that was after insurance covered 50%).
Anyway, I couldn't tell you since you have different insurance. I will say that getting a prescription for the testosterone and needles and doing the injections yourself at home will be cheaper. The injections will avoid all the mess and hassle of the gels and you'll only have to do it once per week or so. Ask your doctor about it.