Hi, This is worth doing a lot of research on. After doing my research and soul searching because we are definitely not made of money either, we just moved my 5yr old son who is 41 lbs and 43in to a booster, but it wasn't cheap. We saved up for it. It is VERy safe on the other hand. I would not put my child (especially a 3 yr old that is likely to unbluckle himeself while you are driving) in to a seat belt only booster for quite a bit longer. He is also too small to be safe in anything but a 5 pt. harness. We bought ds a Britax Frontier. They retail for 279.00 but I got it for 250.00 incl. shipping on ebay. He can sit in it with the 5 pt. harness until he's 80lbs then it becomes a belt positioning booster to 100 lbs. I seriously doubt he'll be in it that long-he'd be like 12! LOL It also has a tether strap to keep it securely fastened to the car's seat. Since your son is a little bigger than mine and 2 yrs younger, you would also probably be able to use this booster for you daughter. So it would save you that way. If you go to different internet stores and look at the customer reviews, you will see several done by parents that are also professional car seat installers. They give the Britax Regent (another Britax booster) and Frontier high praises.
To me this is just one of those things that it is worth spending the money on. And believe me I am ALL about saving maoney and bargain shopping-even if it wasn't the necesseity that it is for us.
Good luck. I hope you can find a way to afford something super-safe for your precious son!
ps. Also, be sure to keep your dd rear facing. I know many folks here have said that she should be rear facing to 1 yr and 20lbs. but that is actually an old recommendation. The new recommendation is that children be kept rear facing until the car seat's rear facing weight limit is met (usually between 30-35 lb). If you move your dd to your son's seat and it isn't made to be rear facing, you need to get her a new seat 1st instead. I know it is a pain, but you'd never forgive yourself it something happened to one of your precious babies over a couple hundred bucks.