As you haven't told us what specific laptop you have, it is not possible to give you disassembly directions.
Most laptops must be disassembled in a specific way, often with specialized tools, to avoid damage -- which may happen anyway. This is one reason shops charge so much -- they have to buy the manuals and tools, and frequently the training. Then if anything goes wrong, they have to replace whatever broke. Just opening a laptop can be a risky task.
Also, on some laptops the HD can just be popped out with a button, or maybe a single screw. On others, you have to take it completely apart -- case, keyboard, motherboard... Sometimes you must remove heat sinks or heat pipes which are glued in place, without so much as bending them. (My NEC Ready is one of those.) And then replace it all afterwards, of course. Itty-bitty screws get stripped, or just dropped and lost. A few models can't
be reassembled without new pieces. Which you'd better already have in stock. Sometimes you have to clean out crud that's been spilled into the computer. Sometimes the crud (especially beverages) has already damaged the circuit board, or a connector, and just moving it will cause it to fail. Permanently.
Some arbitrary drives won't work in some laptops. In that case, the customer isn't going to be happy paying for your time and trouble discovering this fact. Sometimes the computer or drive fails shortly after installation, for reasons entirely unrelated to the work done. The customer will never believe this, and you'll have to eat the cost of the repair.
So you typically have up-front costs, quite a bit of labor, and the risks. $70 is actually quite reasonable in the USA, which is where it sounds like you are. Anything much less, and either they expect to sell you something else, or they probably can't afford to guarantee the work.