Its not so much a question of buying what you can afford, its also a question of what can you afford to maintain if your on a tight budget in the long term. I have my winter bike with"Sora" and to replace the parts, its as cheap as chips and well within my budget and can just buy the parts when I need them, but my main road bike is a mixture of 105 and ultegra, and the cost to replace parts, is well out of my budget so whenever I have to replace chain rings and cassettes etc, then I have to make savings elsewhere to afford the parts.
Its one thing buying an expensive bike, but can you afford to maintain it to its original specification.
But in addition, I have just done a very quick search on the price difference between the two group sets and it is very variable, some sites have one dearer than the other and also vice versa, so in reality I feel, in this case it is a case of getting the better quality group set. I know that the latest 105 group set, 2011, is far cheaper by 9% than the previous 105, so that may be why there is so much variation in price, some sites have the new, hence cheaper, and some may still be selling the old.
If you do a search on line for "tiagra vs 105" there is plenty of info