Britax Diplomat on Closeout at Diapers.com, possibly $135

Diapers.com has the Britax Diplomat convertible car seat for $170 (reg. $260).
They price match, so go HERE to get info on how to price match. You can email them this link to bring the price down to $150. They usually do this within about 5 minutes.
Same deal with the Roundabout!! Search at the AlbeeBaby link above and it is $149 there!

Then, add a package of diapers to your order and use coupon code SEARCH10. (you can use any diapers--even Earth's Best, etc.) This saves $10 for orders over $49. (depending on the price of the diapers you add, they could be free)

Plus, go through Bing for cashback!! It should be 10% for carseats, so another $15 off!
Bing can be tricky, so you may want to take screenshots when you go through Bing to Diapers.com, to prove you went through their link. I did this exact same process when I ordered a carseat last year and I did receive the cashback. However, my experience with Bing has been hit or miss lately.

All this plus free shipping! Remember, your results may vary.

Also, after you order, go and print this rebate form. It is for a free subscription for Parenting, HOWEVER, and the bottom there are instructions for submitting it for a $14.97 rebate instead! (I actually submitted for this rebate awhile ago with Amazon, but it hasn't come yet. Here's hoping!)

Of course all of these tips would work on any purchase at diapers.com!

Thanks SD

Side note: When I got my closeout carseat last year, it was already two years old--it had been in a warehouse that long. I called diapers.com and they took another $25 off the price--after asking for a manager. However, not everyone ordering those carseats got older models. Just keep that in mind. Also, the more you fiddle with an order, the more unreliable Bing becomes. Hopefully it would all go through without a hitch!