The cost is the fuel. It takes an insane amount of energy to leave Sol. The voyager probes were able to save 30km/s worth of Delta-V by performing a ton of gravity assists during the "Grand Tour" launch window planetary alignment that occurs once every 175 years.
For context, getting something to low earth orbit is about 9.4km/s of Delta-V. From LEO to mars is another 4.3 km/s of Delta-V.
Getting from LEO to any solar escape trajectory adds is about 18km/s.
As you can see, 30km/s of fuel savings is extremely significant, and that required the 175 year Grand Tour alignment.
For context, getting something to low earth orbit is about 9.4km/s of Delta-V. From LEO to mars is another 4.3 km/s of Delta-V. Getting from LEO to any solar escape trajectory adds is about 18km/s.
As you can see, 30km/s of fuel savings is extremely significant, and that required the 175 year Grand Tour alignment.