A reflective vest is an extremely effective measure. IF in a situation where it works, it's one of the best measures around. However, it is NEVER sufficient alone. If an approaching car's headlights are not hitting it, it's not lit up. This can happen on corners and curves for instance.

"In the dark" means various things, you should elaborate. Riding at night in a city with street lighting may mean that you just need "be seen" lighting, a modest front and rear flasher may be enough. I ride in pitch black conditions in rural areas without even houses for big stretches, through woods on 50 to 60 MPH roads. I need a lot of light, typically I ride with 500 lumen front lights ($30 to $50).

It also depends on the drivers in your area. On the roads I have ridden for the last 12 years, I felt OK with a strong rear light of about 60 lumens (about $30), heck I had a 30 lumen rear light up until the last 2 or 3 years and I was OK. However I will soon be riding on a road known for distracted drivers, and after having checked the road out, I went out and spent $250 on a rear light that's bright enough to light the bushes on fire, and if I still get close calls, I'm willing to buy police lighting solutions and/or mount lights on vertical poles so that more than just the car behind me can see it.

For most people, a $50 or so lighting package and a $5 vest from Ikea or Harbor Freight will be more than sufficient.

I have spent probably $1000 over the years if you go all the way back to years before LEDs were bright enough to see by and I was using halogen and HID lighting.