I wouldn't be so harsh on the Sentry. Some of them are pretty decent low budget safes. An experienced safecracker will defeat them easily as will a cutting torch, but that's true for any safe!
I take from the post that either economical, light, or expedient security is necessary. You can't always get a 700 pound safe just because that's what you want, even if you have the cash for it.
Basically what are you looking for, a storage locker that will basically keep honest people out, or a low end RSU that will actually impede a casual crook? The former will possibly cost less than $100 and the latter will cost at least $400.
I'm all for the best safe you can afford and find the real estate for, but I've been in police stations and seen the police store their weapons in the cheap gunlockers day in, day out. Things like that have a place and they're a damn sight better than nothing.
If nothing else, it would be prudent to get one just for temporary and save a little money here and there until something else could be bought.
Also, look in the paper and such. Sometimes a used business safe comes up for sale that is big enough to hold guns and they're fairly secure relatively speaking.
I'm often raked over the coals for my opinion on this matter, but I honestly feel without building a special room into the structure itself, you can throw your whole paycheck into weapons storage and still come up short. By all means get something, don't make it easy and quick for some idiot to steal your guns quickly, but it's a simple matter of how good is the theif. Thankfully I do think most thieves that would be smart or resourceful enough to defeat most of these type containers would typically be looking for better scores.
Be honest with yourself about what you really hope to accomplish and how much you can really afford (more than you think probably).
I've a middle of the road unit myself right now, and plan to upgrade to bigger and better because I know I'll move some day relatively soon.