Don't look at the seat as the cause of your problems--it's the symptom. Here are the things you need to look at
1- really good bike shorts. $80-100 isn't unreasonable, the more you spend the better they are, especially for a bigger guy/gal.
2- proper position. If you're sitting upright (and not clipped in) then your rear will be supporting almost all of your weight. If you get clipless pedals and are anchored to the bike that'll take some of the weight off your rear. Additionally a road bike (or more aggressive front end on a mt bike/hybrid will put more weight on your arms, further easing your rear.
3- proper width-- you want your "sit bones" to be centered on the pads of your seat, which, by the way, shouldn't be too cushy. If you sink down too far on your seat you'll push the middle of the seat into your VERY sensitive middle section, which is FAR more uncomfortable than a slightly hard seat.
4- experience... your skin around your sit bones will callous and become used to supporting your weight after awhile, as in, weeks.
5- stand and pedal periodically to allow for fresh bloodflow and to give your rear a break.
I'm 40 pounds heavier than you and these tips all work for me--I've completed 2 centuries this year (including 1 on a mountain bike up and down mountains) and, while I'll be uncomfortable, it's not unbearable. But before I learned these things I was miserable every ride.
Good luck