Reefing on a tight budget is most difficult. If you figure in the startup costs which can be maintained at a minimal level, the running costs must be considered.
Also, you will run into issues trying to reef on a budget. Just figure the initial start up costs. Since you have freshwater tanks, I assume you have most of the hardware needed... Tank, heater, and possibly a canister filter.
You will need new substrate, rocks, and the most expensive component, lights.
Reef lights are high priced items. Your FW lights won't work. Do not fall into the trap of going cheap here. Buy the BEST lights you can. This is an area where it really is a matter of buy it once and be done. Cheap black box lights may work for a while, but you will quickly tire of their lack of controllability and want to upgrade.
Aside from lights, all of your FW hardware will transfer over.
If you decide to run a sump, do some homework on whatever tank you intend to use. Make sure it can be drilled for the drains and returns. Make sure you can fit a sump in your stand. There are Hang on Back overflows that can do the job, but after using one for the last 7 months, I would not recommend them.
If you plan a larger tank, 40g or above, you will probably want to invest in a good RODI water filter. These units can be gotten for a reasonable price on Amazon, but understand that filters need to be replaced as they age. The larger tank you have, the more water you need. More water means faster replacements of filters.
Salt is another running cost. there is an unending debate as to what salt is best. Most of us go with Instant Ocean Reef Crystals. It is the least expensive option for a reef system.
Rocks are something else to consider. Your FW rocks will NOT transfer over. You will need to get new rocks. The price range here is wide depending... Basic dry rock is the least expensive option, but it should, (but not required) be cured for 4-8 weeks before using it in the system.
A more expensive option is the "dry live" rock. This comes from the factory impregnated with all of the cycling bacterial colonies needed to start a tank. It is about 150% higher priced than dry rock. Benefit of this is plug and play. Buy it, place it and you are good to go.
Full "Live Rock" if you live in an area where you can buy it, is the best option but also by far the most expensive. Expect to pay in the range of $20-45 per pound for true ocean live rock, possibly more. It is illegal in California so I don't have a good price for you on that one.
The best advice I can offer to you, is save your money, take your time... You don't HAVE to do this overnight. Buy the best stuff once. Don't fall into the trap of "instant gratification". Getting the cheap stuff may get you started faster, but it will cost you more in the end.
So after all of that... I welcome you to the Dark Side....
